What are the attachment and message size limits for different mailbox providers?
Published 30 Apr 2025
Updated 23 Oct 2026
13 min read
Summarize with
Updated on 23 Oct 2026: We corrected provider size limits and clarified attachment encoding, Gmail clipping and practical message testing.
Most mailbox providers enforce a total SMTP message size limit as well as an attachment upload limit. Personal Gmail allows 25 MB in combined outgoing attachments and 50 MB incoming messages. Outlook.com documents a 25 MB email limit; the often-quoted 20 MB figure is a default in classic Outlook for internet email accounts. Yahoo Mail publishes a 25 MB combined attachment limit, while iCloud Mail limits incoming and outgoing messages to 20 MB. Business account limits depend on the edition and administrator settings, as shown below.
Attachments are MIME encoded inside the message body. Base64 data expands by about 33%, and line wrapping brings the overhead to about 37%. Use a 1.37 multiplier to estimate encoded attachment size, then add the rest of the email. A 25 MB encoded message limit leaves roughly 18 MB for raw attachments in a lean email. This calculation applies to total message limits, not a provider upload limit already measured before encoding.
- Safe default: keep the full email under 10 MB for broad one-to-many sends; for direct attachments, a raw file under 7 MB usually stays inside that budget.
- Upper budget: keep total raw attachments under 14 MB for a confirmed 20 MB message limit, or under 18 MB for a confirmed 25 MB message limit. Smaller corporate limits need a smaller budget.
- Large files: use a secure file link above 18 MB, or sooner when the recipient limit is unknown, even when your sender UI accepts the attachment.
- Marketing emails: avoid direct attachments unless the workflow requires them; use hosted assets and links instead.
Email attachment and message size limits by provider
These are published attachment or message limits, with the measurement basis identified where documented. The smallest applicable limit anywhere in the path wins. A sender can accept a file, then the recipient server can reject the encoded message later. Do not treat every number in the table as an encoded SMTP limit.
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Gmail (personal) | 25 MB combined attachments | 50 MB message | Google Drive link above the attachment limit |
Google Workspace | 25 MB content and attachments; up to 50 MB for Enterprise Plus on the web | 50 MB message; 70 MB for Enterprise Plus | Sending limit is before encoding; receiving limit is after encoding. Administrator settings apply. |
Outlook.com | 25 MB email | 25 MB email | Classic Outlook can impose a separate 20 MB client limit; use OneDrive links for larger files. |
Microsoft 365 (Exchange Online) | 35 MB message by default | 36 MB message by default | Administrators can configure up to 150 MB; client and external-route limits still apply. |
Yahoo Mail | 25 MB published attachment limit | 25 MB published attachment limit | Combined files, not a per-file allowance. Yahoo also documents a 25 MB email cap, so leave room for encoding and content. |
iCloud Mail | 20 MB message | 20 MB message | Mail Drop sends files up to 5 GB through a separate download workflow. |
Proton Mail | 25 MB attachments | 50 MB attachments | Up to 100 attached files; account for encoding overhead. |
Zoho Mail | 20 MB personal message; 30-40 MB paid attachment limit | Up to 40 MB message | Plan and administrator policies apply; Huge Attachments use links up to 1 GB. |
Fastmail | 70 MB message | 70 MB message | About 50 MB in raw attachments after allowing for encoding |
GMX | 50 MB attachments | 50 MB attachments | Recipient message limits still apply. |
mail.com | 30 MB FreeMail attachments; 100 MB Premium message | 30 MB FreeMail attachments; 100 MB Premium message | Premium allowance includes the message body, headers, and attachments. |
Yandex Mail | 25 MB direct attachments | 30 MB message | Yandex Disk link above 25 MB in attachments |
Published limits distinguish attachment allowances from total message size; administrator and client restrictions still apply.
Corporate mail systems add another layer. Microsoft 365 tenants, private Exchange servers, gateways, and content filters can enforce smaller per-user, per-connector, per-domain, per-file-type, or encrypted-message limits. When delivery matters, test the actual path instead of trusting a generic provider number. A 150 MB Exchange Online setting does not guarantee a 150 MB external attachment: Outlook on the web has a 112 MB ceiling at that setting, and external routing adds encoding overhead.
Gmail compose window converting an oversized attachment into a Google Drive link.
Attachment limit vs message limit
The phrase attachment limit is convenient, but SMTP delivery is usually governed by message size. A MIME attachment is part of the message payload. The receiver does not see a 20 MB PDF as only a 20 MB file; it sees the encoded MIME body, headers, boundaries, text content, and other parts. Gmail's published attachment allowance is not automatically reduced by another 37%; check the recipient's encoded message limit separately.
Remote newsletter images add their HTML references to the message, rather than their full file contents. Images embedded as MIME parts, including signature logos, do add their encoded file size. Whether an image appears inline does not determine how it is stored.
Gmail clipping is a separate HTML body size issue, commonly encountered around 102 KB. It hides content behind a "View entire message" link without rejecting the email. Keep the delivered HTML comfortably below that threshold, including tracking markup and the unsubscribe link, and test the final message in Gmail. Shrinking an attached PDF does not fix oversized HTML.
Attachment limit
- User view: the size shown beside a PDF, image, spreadsheet, or zip file on disk.
- UI rule: the webmail or app composer blocks files above its upload threshold.
- Common mistake: treating a 25 MB attachment upload allowance as the same rule as a 25 MB encoded message limit.
Message size limit
- SMTP view: the encoded wire payload after MIME packaging and transport encoding.
- Server rule: the receiving MTA rejects mail above its advertised or configured size.
- Correct habit: budget for encoding overhead before the message leaves your platform.
Encoding mathtext
raw_attachments_mb * 1.37 + other_message_mb = estimated_message_mb 14 MB * 1.37 = 19.18 MB before other message content 18 MB * 1.37 = 24.66 MB before other message content 20 MB / 1.37 = 14.60 MB before allowing for other content 25 MB / 1.37 = 18.25 MB before allowing for other content
That is why raw attachments under 14 MB are a cautious budget for a confirmed 20 MB message limit, such as iCloud Mail, and under 18 MB is a budget only for a confirmed 25 MB encoded limit with a small message body. Neither budget guarantees delivery through a stricter corporate gateway. For deeper detail on encoding and content types, read MIME size effects.
How SMTP advertises the wire limit
If you are diagnosing server-to-server delivery, the SMTP EHLO response often includes a SIZE extension. A positive numeric value declares the largest message size the server advertises, in bytes, including headers and MIME content. SIZE without a number, or SIZE 0, does not advertise a fixed positive maximum. None of these responses guarantees acceptance for an individual mailbox.
Illustrative SMTP SIZE responsetext
C: EHLO sender.example S: 250-mx.recipient.example S: 250-SIZE 25000000 S: 250 8BITMIME # 25,000,000 bytes = 25 MB, or about 23.84 MiB # This is an example, not a live provider limit.
How to read SIZE
A large SIZE value means the MX is advertising a large SMTP transaction size. It does not guarantee that a user can upload a file of that size in the provider UI, that the sender account is allowed to submit it, or that policy filters will accept it.
- Use it for: MTA troubleshooting, direct SMTP testing, and explaining server rejection behavior.
- Do not use it for: promising that a consumer mailbox UI accepts the same attachment size.
- For raw files, add Base64 overhead and the other MIME parts. For an assembled MIME message, measure its bytes without applying encoding overhead again.
This distinction also explains why one recipient accepts a message while another rejects it with a size error. The sender UI, outbound MTA, relay, gateway, and destination MX each get a vote. For the bounce pattern itself, see max message size errors.
What happens when an email is too large
An oversized email does not always fail at the same point. The sender app can block it immediately, the outbound relay can accept it and then receive a remote bounce, or a gateway can strip the file and deliver only the body. Track the exact failure point because the fix differs.
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Submission error | The sender app or outbound relay rejects the message before delivery. | Compress the file or replace it with a link before retrying. |
Recipient bounce | The destination server rejects the encoded message after acceptance upstream. | Log the recipient domain, SMTP code, and encoded size. |
Silent non-delivery | An older or misconfigured gateway accepts the message without producing a clear bounce. | Use logs, recipient confirmation, and a file link for that route. |
Attachment stripping | A security gateway removes the file but delivers the message body. | Use a known-access link for that recipient domain. |
Delay or extra filtering | The message fits the size cap but requires more scanning or queue time. | Reduce embedded content, check file policy, and test the real message. |
Common oversized-message outcomes.
Size-related bounces commonly include 5.2.3 or 5.3.4 with text such as "max message size exceeded". Read the full diagnostic: 552 alone does not identify a size problem and is also used for mailbox quota failures. After a permanent size rejection, compress the file, lower the raw attachment budget, or use a link before resending.
How to reduce attachment size
Reduce the source file before splitting it across messages. Compare the archive's encoded size with the message limit. Security gateways can separately restrict the uncompressed contents of an archive.
- Remove unneeded pages, duplicate images, hidden slides, and revision history before compression.
- Re-export image-heavy PDFs with the screen or web preset, then confirm that text and charts remain readable.
- Resize photos to their intended display dimensions and lower JPEG quality before placing them in a document.
- Replace embedded signature logos in long threads with small hosted images or plain text.
- Use a ZIP file only when it produces a meaningful reduction and the recipient expects that file type.
JPEG, PNG, video, and many modern document formats are already compressed, so placing them in a ZIP file often saves little. Password-protected archives and executable file types can also trigger security controls even when their size is acceptable.
A safe sending policy
For most teams, the answer is not to chase every provider maximum. The better policy is to define a safe maximum email size that works across the common mailbox mix, then make exceptions only for known internal routes or file-link workflows. Large direct attachments also duplicate storage across mailboxes and backups, and repeated attachment-heavy sends can slow queues.
Attachment planning bands
Use total raw attachment size before MIME encoding. These are planning budgets, not guarantees of acceptance.
Broad sends
Under 7 MB
Under 7 MB becomes under about 9.6 MB encoded, leaving some room within a 10 MB message target.
Caution
7-14 MB
Up to about 19.2 MB encoded before the rest of the email; confirm the recipient limit.
High risk
14-18 MB
Needs a confirmed 25 MB encoded message limit and a small body; stricter routes reject it.
Use links
18 MB+
Send a secure file link instead of a direct MIME attachment.
Use stricter targets by document type: invoices and receipts should stay under 1 MB when practical; image-heavy business documents should stay under 5 MB; marketing emails should use hosted assets and links instead of direct attachments. For an essential transactional attachment, include an authenticated customer-portal download link as a fallback.
When a campaign or automated workflow reaches many recipient domains, treat 10 MB as a total message-size target. It is a practical default, not a universal guarantee: a recipient can enforce a lower cap. Confirm the real route before sending a larger one-to-one attachment.
Before an automated workflow goes out with attachments, send a non-sensitive sample through Suped's email tester. Suped receives the test message and checks authentication and sending configuration. Measure the assembled MIME message separately and send a sample to the intended recipient environment to check its size policy.
Email tester
Send a real email to this address. Suped shows a results button when the test is ready.
?/43tests passed
Suped's test checks SPF, DKIM and DMARC results alongside blocklist (blacklist) signals. It does not establish the attachment limit at another provider. If the sender also has DNS or authentication issues, run domain health checks to investigate those separately from a size rejection.
What to do by recipient type
The right limit depends on who receives the mail. Consumer mailboxes reward conservative sizing because the sender rarely controls the recipient policy. Business mailboxes can accept larger mail when both sides are on Microsoft 365 or another configurable platform, but external hops still create failure points.
Consumer recipients
- Keep invoices, receipts, and statements under 1 MB when the document format allows it.
- Use links for video, scan bundles, design files, and image-heavy reports.
- Avoid zips when the recipient mix includes strict security gateways.
Business recipients
- Confirm tenant, mailbox, connector, gateway, encrypted-message, and client limits.
- Larger internal limits do not mean external domains accept them.
- Save SMTP enhanced status codes and recipient domain details.
Cloud links are not the same as attachments
Google Drive links, OneDrive links, iCloud Mail Drop, and similar file-link workflows keep the file outside the SMTP payload. The email carries a link, so check file access permissions and recipient trust as well as message delivery.
- Check access so external recipients can open the file without extra account friction.
- Check expiry because Mail Drop downloads expire after 30 days and other link policies vary.
- Check trust because unexpected download links can look risky to recipients and security filters.
For PDFs, file type and file size both matter. A scanned PDF can be ten times larger than a text-based PDF, and password-protected or scripted files trigger extra security handling. The practical guidance is covered in PDF attachment practices.
How Suped supports message testing
Suped's email tester receives a message sent through your normal provider and checks the authentication and sending setup used for that message. Use it when an attachment complaint needs investigation alongside SPF or DKIM failures. Use your sending logs and the recipient's diagnostic response to establish a size rejection.
Email tester sample report showing total score, email preview, issue summary, and per-section results
If you are staging a DMARC rollout, add message testing to the release checklist. Suped's DMARC monitoring tracks authentication through reports from participating receivers. Measure MIME size and test recipient acceptance separately; DMARC aggregate reports do not report attachment size or individual size-related bounces.
How to calculate a safe attachment size
When a workflow needs an exact answer, work backward from the smallest confirmed encoded message limit across the sender, relay, and recipient. For unknown recipient environments, keep the assembled email under 10 MB or use a file link. Use the 14 MB and 18 MB raw attachment budgets only when the route accepts 20 MB and 25 MB messages respectively.
- Identify each applicable message limit and any separate attachment upload limit.
- Subtract the other message content from the smallest encoded message limit, then divide the remaining space by 1.37.
- Apply any lower per-file or combined attachment limit, and leave extra room for headers added during delivery.
- Export the assembled message as an .eml file with SMTP CRLF line endings and count its bytes before submission. Send the actual template and attachment through the intended route.
Planning examples with a 1 MB allowance for other message contenttext
(25 MB encoded limit - 1 MB other content) / 1.37 = 17.52 MB raw budget (20 MB encoded limit - 1 MB other content) / 1.37 = 13.87 MB raw budget (10 MB message target - 1 MB other content) / 1.37 = 6.57 MB raw budget
Count bytes in an assembled MIME message on macOS or Linuxbash
wc -c < message.eml
Use the byte count of the final MIME message when available; the 1.37 multiplier is an estimate for Base64 attachments. MB is 1,000,000 bytes and MiB is 1,048,576 bytes, so compare exact bytes when a file is close to a limit. Extra routing headers and security packaging still need headroom.
