How to fix SMTP 451 and 554 local processing errors for .eml messages
These appear when a mail server cannot process a message or an uploaded .eml file. A 451 is temporary and the sender should retry, while a 554 is a permanent rejection. With .eml files both usually mean a broken MIME structure or a security filter. Jump to your situation below or work through the methods in order.
By Neeraj Singh ~8 min Updated Jun 2026 91% found this helpful
Error message
451 4.3.0 Temporary server error. Please try again later. PRX5 / 554 5.6.0 Message could not be processed, MIME content is corrupted.
Summary
SMTP 451 and 554 are two ends of the same family. 451 is a transient error: the server could not process the message right now, so the sending server should retry automatically over the next minutes or hours. 554 is a permanent failure: the transaction was rejected and will not be retried. When these involve an .eml file, the cause is usually that the message's MIME structure is broken, for example missing or mismatched part boundaries, malformed headers, or bad transfer encoding, or that a content, spam or anti-malware filter on the receiving server blocked it. Start by reading the response code and the server log, retry a 451, and for a 554 repair or re-export the .eml so its MIME structure is clean, reduce the size if it is too large, and check the receiving server's filtering rules.
What this error means
An SMTP server replies with a status code for every message it handles. A 451 is a 4xx transient reply, meaning try again later, the problem is expected to clear on its own. Sending servers queue the message and retry on a schedule, so a one-off 451 often resolves without any action.
A 554 is a 5xx permanent reply, meaning the message was rejected and will not be retried. With .eml files this usually points at the message itself: a corrupted or malformed MIME structure the server cannot parse, or content that a security or policy filter refused. Fixing the file or the filter is what clears a 554.
Common causes
The .eml has corrupted or malformed MIME parts.
MIME part boundaries are missing or do not match.
Headers or the Content-Transfer-Encoding are invalid.
The message exceeds the receiving server's size limit.
A content or spam filter blocked the message.
Anti-malware quarantined an attachment in the .eml.
The server was temporarily overloaded or out of resources (451).
Expert insight
“The first thing I check is the digit: a 451 is the server saying not now, try again, and most of the time the queue sorts it out by itself. A 554 is the server saying no, and with .eml files that almost always means the MIME is broken or a filter slammed the door. People waste hours hand-editing the file when the cleanest move is to re-export the .eml from the mail client so the boundaries and encoding are correct. And always read the server log, it usually names the exact rule or the part that failed.”
Manager, Tech Support & Operations · 19+ years fixing Windows and system errors
✓ How to fix it
Method 1
Tell a 451 from a 554
1Read the response code. A 451 (4xx) is temporary, the sender should retry. A 554 (5xx) is permanent, you must fix something and resend.
2Note the extended code and text after it, for example 5.6.0 or 4.3.0, which hint at the reason.
3This decides whether you wait or repair.
Method 2
For a 451, wait and retry
1A 451 usually clears on its own as the sending server retries on its schedule.
2If you are sending manually or from an app, wait a few minutes and send again.
3If 451 persists for hours, treat it like a 554 and investigate the message and the server.
Method 3
Inspect and repair the MIME structure
1Open the .eml in a text editor and check it has valid headers, a correct Content-Type, matching multipart boundaries, and a sane Content-Transfer-Encoding.
2Broken or mismatched boundaries are the most common cause of a 554 on .eml.
3Repair the structure, or better, re-export the message cleanly (next method).
Method 4
Re-export the .eml cleanly
1Rather than hand-editing, open the original message in a mail client and save it again as a fresh .eml.
2A clean export rebuilds the MIME structure correctly.
3Resend the new file.
Method 5
Reduce the message size
1If the message is large, it may exceed the receiving server's limit and be refused.
2Remove or shrink large attachments, or send them another way.
3Keep the total under the destination's stated size limit.
Method 6
Check content and security filters
1On the receiving server, review the content, spam and anti-malware filters and any transport rules.
2A false positive can reject a clean message, so allow the sender or adjust the rule if appropriate.
3Quarantined attachments are a common 554 cause.
Method 7
Read the mail server logs
1The server log records the exact reason and often the rule that rejected the message.
2Match the extended status code to the log entry to confirm the cause.
3Fix the named issue, then resend.
Do not panic over a single 451, it is temporary by design and the sending server will keep retrying. A 554 is the one that needs action, and on .eml files it is usually a broken MIME structure or a filter, not a network problem. Re-exporting the message cleanly fixes most structural 554s without any manual MIME editing.
Frequently asked questions
What is the difference between SMTP 451 and 554?
451 is a temporary error, so the sending server retries automatically. 554 is a permanent rejection that will not be retried until you fix the message or the server-side rule.
Should I resend after a 451?
Usually you do not need to. Mail servers queue and retry 451s on their own. If you are sending from an app, waiting a few minutes and retrying is fine.
Why does my .eml cause a 554?
Most often because its MIME structure is broken, such as mismatched boundaries or bad encoding, or because a content or anti-malware filter on the receiving server blocked it.
How do I fix a corrupt MIME .eml?
Re-export the message from a mail client as a fresh .eml rather than hand-editing it. A clean export rebuilds the headers, boundaries and encoding correctly.
Could a filter be causing the 554?
Yes. Content, spam and anti-malware filters or transport rules on the receiving server can reject a clean message. Check the logs and allow the sender if it is a false positive.
How do I find the exact reason?
Read the receiving mail server's log. It records the rejecting rule and the extended status code, which together tell you precisely why the message failed.
Still not working?
If a 554 persists after re-exporting the .eml and ruling out size and filters, the receiving server may have a strict transport rule or a reputation block on your sending domain or IP. Check your SPF, DKIM and DMARC alignment and your IP reputation, and contact the receiving postmaster with the log entry. You can also submit your error to us for a tailored fix.
