Summary
Message size limits vary across different mailbox providers. While some providers like Gmail and Yahoo Mail offer a 25 MB limit, others like Outlook.com restrict attachments to 20 MB (with Microsoft Exchange configurable up to 150MB). Some, like IONOS, have limits that depend on the specific email package. GMX offers 50 MB total but advises smaller attachments due to encoding overhead. Zoho Mail provides 20MB (standard) or 25MB (paid). Experts emphasize that limits apply to the total message size, including encoding, not just attachment size. It's generally recommended to keep attachments smaller, under 2MB-10MB for better deliverability, and to use cloud storage services for larger files. Base64 encoding can increase attachment size by roughly 33%. Many corporate email systems have stricter limits (10MB), and ISPs generally suggest keeping total message size under 15MB.
Key findings
- Varying Limits: Attachment and message size limits vary significantly between email providers and systems.
- Total Message Size: Limits apply to the total message size on the wire, including headers and encoding, not just the attachment file size.
- Encoding Impact: Base64 encoding, commonly used for attachments, increases the message size by approximately 33%.
- Common Limits: Typical limits range from 10MB to 25MB, although some providers offer higher limits (up to 50MB or more for Exchange).
- Deliverability Impact: Large attachments can negatively impact email deliverability, potentially triggering spam filters and causing bounces.
- Corporate Restrictions: Many corporate email systems have strict attachment size limits, often around 10MB, due to security concerns.
Key considerations
- Account for Encoding: Remember to factor in base64 encoding overhead when calculating total message size.
- Consider Recipient Limits: Be mindful of the recipient's email provider's limitations, even if your own provider allows larger attachments.
- Prioritize Deliverability: Opt for smaller attachment sizes (under 2MB-10MB) to maximize deliverability and avoid triggering spam filters.
- Utilize Cloud Storage: Use cloud storage services (e.g., Google Drive, Dropbox) and share links instead of directly attaching large files.
- Optimize Attachments: Compress files and optimize images to reduce their size before attaching them.
- Check Specific Limits: Consult your specific email provider's documentation or support for accurate and up-to-date size limits, as they can vary by plan or service.
What email marketers say
8 marketer opinions
Email attachment size limits vary across mailbox providers and can significantly impact deliverability. While some providers allow up to 25MB, it's generally recommended to keep attachments smaller, typically under 2MB to 10MB, to avoid issues. Using cloud storage services and linking to files is a widely suggested alternative for sharing larger content.
Key opinions
- Varying Limits: Acceptable attachment sizes differ across email clients, ranging from 10MB to 25MB.
- Deliverability Impact: Large attachments negatively affect email deliverability, potentially triggering spam filters or causing bounces.
- Corporate Restrictions: Corporate email systems often have stricter limits, around 10MB, due to security concerns.
- ISP Recommendations: ISPs generally suggest keeping total message size (including attachments) under 15MB.
- Encoding Overhead: Encoding of attachments (e.g., base64) increases the final message size.
Key considerations
- Attachment Size: Prioritize smaller attachment sizes (under 2MB - 10MB) for better deliverability.
- Alternative Methods: Utilize cloud storage services (Dropbox, Google Drive) and share links instead of attaching large files.
- Content Optimization: Optimize images and compress files before attaching them to reduce their size.
- Recipient Capacity: Consider the recipient's email provider's limitations when sending attachments.
Marketer view
Email marketer from Constant Contact recommends keeping email attachments below 5 MB for optimal deliverability and user experience.
27 Aug 2022 - Constant Contact
Marketer view
Email marketer from Reddit user u/EmailPro explains that many corporate email systems have a strict 10MB limit due to security concerns, and exceeding this will bounce the email.
21 Feb 2024 - Reddit
What the experts say
5 expert opinions
Message size limits are imposed on the entire message 'on the wire,' not just the attachment size. To ascertain size limits, the `SIZE` response to the `EHLO` command provides the accepted encoded bytes. Base64 encoding of attachments increases their size by roughly 33%. Limits below 10MB are uncommon.
Key opinions
- Message Size Limit: Limits apply to the total message size on the wire, not specifically attachments.
- EHLO SIZE Response: The `SIZE` response to the `EHLO` command reveals the server's acceptable encoded byte limit.
- Base64 Encoding Overhead: Base64 encoding inflates attachment sizes by approximately 33% (multiply by 1.34 for estimation).
- Minimum Size Limit: Message size limits below 10MB are not commonly encountered.
Key considerations
- Calculate Encoded Size: Account for the base64 encoding overhead when calculating the total message size.
- Use EHLO for Limit: Utilize the `EHLO` command to determine the specific size limits of the receiving server.
- Stay Below 10MB: Aim to keep total message size under 10MB to ensure wider compatibility.
Expert view
Expert from Email Geeks shares that they haven’t seen a message size limit lower than 10 megabytes recently.
22 Dec 2021 - Email Geeks
Expert view
Expert from Email Geeks explains that if an attachment is base64 encoded, you can multiply the attachment size by 1.34 to estimate the byte cost on the wire.
7 Sep 2023 - Email Geeks
What the documentation says
6 technical articles
Message size limits vary across mailbox providers. Gmail and Yahoo Mail offer a 25 MB limit. Outlook.com restricts attachments to 20 MB, while Microsoft Exchange accounts have a default of 10 MB (configurable up to 150MB). IONOS limits depend on the specific package. GMX offers 50 MB total but advises smaller attachments to account for encoding. Zoho Mail provides 20MB for standard and 25MB for paid plans. Cloud storage solutions are generally recommended for larger files.
Key findings
- Gmail and Yahoo: Gmail and Yahoo Mail have a message size limit of 25 MB.
- Outlook.com: Outlook.com limits attachments to 20 MB.
- Microsoft Exchange: Microsoft Exchange accounts default to 10 MB (configurable up to 150 MB).
- IONOS Variability: IONOS limits are dependent on the specific email package.
- GMX Total Limit: GMX offers a 50 MB total limit, accounting for encoding overhead.
- Zoho Plans: Zoho Mail provides 20 MB (standard) or 25 MB (paid) attachment limits.
Key considerations
- Cloud Storage: Utilize cloud storage solutions for sending files exceeding provider limits.
- Encoding Overhead: Account for encoding overhead when calculating attachment sizes, especially with GMX.
- Specific Package Limits: Consult your specific email package details for accurate size limits (IONOS).
Technical article
Documentation from Microsoft Support explains that Outlook.com has an attachment size limit of 20 MB. For Microsoft Exchange accounts, the default limit is 10 MB but can be configured by the administrator up to 150MB.
11 Sep 2021 - Microsoft Support
Technical article
Documentation from Google Support explains that Gmail has a message size limit of 25 MB. If you need to send larger files, use Google Drive.
18 Feb 2022 - Google Support
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