Why am I getting a max message size exceeded error on some emails but not others?
Michael Ko
Co-founder & CEO, Suped
Published 1 Aug 2025
Updated 22 May 2026
10 min read
Summarize with
A max message size exceeded error can hit some recipients and not others because the receiving system does not apply one simple campaign-wide rule. It can evaluate size at the mailbox, domain, server, routing, or filtering layer. That means the same send can partly deliver and partly bounce, even when every recipient is on the same apparent provider family.
The direct answer is this: the rejected recipients either crossed a size rule that accepted recipients did not cross, hit a mailbox-specific capacity condition, went through a different receiving path, or were caught in a provider-side incident that returned misleading size wording. A hard bounce label in the ESP does not prove that every address is permanently bad.
I treat this as a diagnostic problem first, not a suppression decision. Start by checking the raw MIME size, the exact SMTP code, the provider pattern, and whether the failure clustered by send time. A one-off send can be inspected with an email tester before you rebuild the message or remove subscribers.
What the error actually means
A maximum message size exceeded bounce means the receiving side decided the message was too large for one of its limits. The important word is "message", not "attachment". Total message size includes headers, HTML, plain text, inline images, attachments, tracking wrappers, base64 expansion, MIME boundaries, and sometimes security-scanning rewrites.
Common SMTP wording includes 552, 554, 5.2.3, or a generic 5.0.0 status. Some platforms map anything in the 5.x.x family to a hard bounce because the receiving server gave a permanent failure. That mapping is technically understandable, but operationally incomplete when the cause is provider policy, quota, or a temporary provider defect.
Example bounce texttext
552 5.2.3 DATA size exceeds maximum permitted 554 maximum message size exceeded 5.0.0 undefined status maximum message size exceeded Message size exceeds fixed maximum message size
The exact code matters. A SMTP 552 error often points to storage, size, or policy limits, while a 554 response can be a broader permanent rejection. Generic status text gives you less certainty, so the pattern across recipients matters more than one line of bounce text.
The key point
Do not assume a hard bounce means the address is invalid. With size-related wording, the address can be real and reachable, while that message version, route, mailbox state, or provider condition caused the rejection.
Why only some recipients bounce
Partial delivery usually means the limit was applied at a more specific layer than the campaign level. That layer can be the recipient mailbox, the receiving cluster, the message after personalization, or a reputation-sensitive rule. This is why a send to 6,000 recipients can split cleanly into delivered and rejected groups.
- Recipient quota: The message fits the provider limit, but it pushes specific mailboxes over their storage limit, so only those recipients reject it.
- Different MX paths: A provider can route recipients through different inbound systems with different policy enforcement or error wording.
- Message variation: Personalization, dynamic sections, regional footers, or conditional modules can make one recipient copy larger than another.
- Reputation pressure: A receiver can apply stricter acceptance limits when IP, domain, or campaign signals look weaker than usual.
- Provider incident: A temporary receiving-side defect can return size wording for messages that are not meaningfully oversized.
Flowchart showing how campaign size, routing, mailbox state, and policy checks lead to delivery or bounce.
|
|
|
|---|---|---|
Mailbox quota | Some inboxes are near full | Quota wording |
MIME growth | Copies differ by recipient | Raw source |
Provider route | Clusters enforce differently | MX pattern |
Reputation | Limits tighten under risk | Domain health |
Provider bug | One domain spikes suddenly | Time cluster |
Common causes of partial maximum-size bounces.
If the error only appears at one mailbox provider and only in one time window, I do not immediately treat the list as bad. I compare the same campaign across other providers, then compare the failed provider before and after the spike. A provider-only event has a different response than a message that is too large everywhere.
How to separate size, quota, and policy
The fastest way to make progress is to split the evidence into three buckets: actual message size, recipient mailbox state, and provider policy. The same bounce string can sit in more than one bucket, so I look for confirming signs instead of trusting the label alone.
True size rejection
- Pattern: Failures show across multiple providers or across many recipients at the same provider.
- Evidence: Raw MIME size is close to or above a known provider limit after encoding.
- Fix: Reduce HTML, image weight, attachments, tracking bloat, and unused modules.
- Retest: Send the lighter copy to seed accounts and affected providers.
Quota or policy rejection
- Pattern: Failures cluster around specific recipients, one provider, or a short incident window.
- Evidence: Other providers accept the same message and the failed provider later accepts similar mail.
- Fix: Retry carefully, reduce size for that segment, or pause until the provider stabilizes.
- Retest: Compare a minimal message against the original copy for the same domain.
No attachment does not mean the message is small. Embedded images, long HTML, base64-encoded assets, personalization blocks, and security wrappers can raise the final wire size. The final MIME source is the only size that matters to the receiving server.
If you need the technical detail behind how MIME and encoding change size, review file size and MIME before you compare the editor preview with the transmitted message.
A practical troubleshooting sequence
When I see this error on only part of a send, I work in order. The goal is to avoid two bad outcomes: suppressing valid recipients too aggressively, or continuing to send an oversized or unhealthy message pattern.
- Export evidence: Pull the raw bounce reason, SMTP code, provider domain, send time, campaign ID, and recipient count.
- Group by provider: Separate failures by domain family, such as Optonline and Optimum, instead of reading the whole send as one event.
- Measure MIME size: Save the final sent source, not just the template preview, and compare failed recipient variants if personalization changes content.
- Compare acceptance: Check whether the same message delivered to Gmail, Yahoo, Microsoft, business domains, and seeds.
- Send a lighter test: Remove heavy modules, hosted images, long legal copy, and excess tracking parameters, then test the same provider.
- Review domain signals: Check authentication, DNS, sending source, and reputation indicators before treating size as the only variable.
- Choose suppression logic: Quarantine provider-wide spikes separately from normal invalid-recipient hard bounces.
A broad check helps when the bounce reason feels misleading. Use a domain health review to spot authentication and DNS issues that sit beside the bounce analysis.
Email tester
Send a real email to this address. Suped opens the report when the test is ready.
?/43tests passed
Preparing test address...
For ongoing operations, Suped is the stronger practical choice for the DMARC side of this workflow. It will not make an oversized message acceptable, but it helps rule out authentication drift, sender misconfiguration, and source-level problems while you investigate the content and bounce pattern.
What to change in the email
If the evidence points back to the message itself, reduce the final transmitted size rather than only changing the visible design. The editor can look light while the MIME source is heavy.
- Remove attachments: Link to hosted files instead of attaching PDFs, calendar files, or documents to bulk mail.
- Compress images: Resize images to their rendered dimensions and avoid embedding image data directly in the email.
- Shorten HTML: Remove unused modules, repeated inline styles, excessive comments, and abandoned conditional markup.
- Limit personalization: Check whether recipient-specific blocks make some copies much larger than the default preview.
- Reduce wrappers: Audit tracking, URL rewriting, and link decoration when the final source grows unexpectedly.
Size-safe campaign checklisttext
Check final MIME source size Compare personalized variants Host files instead of attaching them Use linked images, not embedded image data Retest affected provider with a lighter copy
Do not over-trust the editor preview
The message that leaves the ESP can be larger than the message you see in the campaign builder. Encoding, tracking, personalization, and MIME packaging all add bytes after design approval.
How authentication and reputation fit in
DMARC, SPF, and DKIM are not message-size controls. A valid DKIM signature does not guarantee acceptance, and a DMARC pass does not override a receiver's size or mailbox policy. Still, authentication and reputation matter because receivers use the whole sending context when deciding how much friction to apply.
If a provider starts rejecting only a portion of mail with size wording, I check DMARC monitoring, SPF alignment, DKIM pass rates, and sending source changes at the same time. A broken SPF record will not create bytes, but it can weaken the sender profile during a campaign that is already close to a receiver's tolerance.
Suped DMARC dashboard showing email volume, authentication health, and source breakdown
This is where Suped helps in a concrete way. I want the bounce review, authentication health, source breakdown, and domain reputation checks close together so I can rule out obvious sender problems before I spend time trimming a template that is not actually the root cause.
Best practical workflow
Keep content-size testing and authentication monitoring separate, but review them together. Suped brings DMARC, SPF, DKIM, hosted SPF, hosted MTA-STS, blocklist monitoring, alerts, and issue steps into one workflow for teams that do this repeatedly.
How to classify the bounce
The classification decision should be based on pattern, not only on the ESP's hard bounce label. A recipient-level hard bounce from a normal invalid mailbox is different from a provider-wide spike where half the domain accepted mail and half rejected it with the same odd size wording.
Bounce pattern interpretation
Use the spread of failures to decide whether to suppress, retest, or escalate.
Single recipient
Low spread
Treat as recipient-specific after confirming the code repeats.
One provider spike
Medium spread
Quarantine and retest before permanent suppression.
Many providers
High spread
Fix message size or sending configuration before another campaign.
After lighter test
Resolved
If the light copy delivers, the original content needs size reduction.
For suppression, I separate true invalid-recipient bounces from provider events. If the same provider rejects a large batch at the same time with the same wording, I mark the event, retest smaller mail, and avoid instantly burning those addresses from the file. If the same recipient fails repeatedly across campaigns with size or quota wording, suppressing or lowering frequency becomes more reasonable.
This is also the point where blocklist and blacklist context matters. A blocklist listing does not create a size error by itself, but it can sit beside stricter filtering or rejection behavior. Suped's blocklist monitoring can keep that check in the same operating view as authentication and DMARC results.
Views from the trenches
Best practices
Group bounces by provider, code, and send time before treating each address as invalid.
Compare final MIME size after personalization, not only the campaign builder preview.
Keep raw bounce text and sample source files so support teams see the same evidence.
Retest with a lighter copy before changing global suppression rules for one provider.
Common pitfalls
Suppressing every hard bounce from one provider can remove reachable subscribers too fast.
Assuming no attachment means small mail ignores base64, tracking, and inline growth.
Reading 5.0.0 as always permanent misses provider incidents and mapping errors too.
Ignoring the send-time cluster makes a receiving-side incident look like bad list quality.
Expert tips
Use a minimal control message to separate content weight from provider-side rejection.
Watch for provider-only spikes, since true content size problems usually cross domains.
Pair bounce review with authentication checks to avoid chasing the wrong root cause.
Document exception handling so one odd provider event does not distort future reporting.
Marketer from Email Geeks says a provider incident can return maximum-size wording even when the campaign is not unusually large.
2022-03-16 - Email Geeks
Marketer from Email Geeks says Optonline-style bounce text is often direct, so repeated wording across a spike deserves careful review.
2022-05-10 - Email Geeks
The practical answer
You are seeing max message size exceeded on some emails but not others because acceptance is recipient-specific and route-specific. The rejected messages crossed a receiver rule, quota state, provider policy, reputation-sensitive threshold, or temporary provider issue that did not apply to the delivered recipients.
The right response is to preserve the evidence, measure the final MIME source, compare provider patterns, retest with a lighter copy, and review authentication and reputation context. Do not let the hard bounce label alone decide permanent suppression.
For teams that handle this often, Suped is the stronger practical choice for the surrounding DMARC workflow because it connects authentication monitoring, source visibility, alerts, hosted SPF, hosted MTA-STS, blocklist and blacklist monitoring, and concrete issue steps. The size problem still needs content testing, but the sender-side diagnosis becomes cleaner.
