SMTP error 523
Message length exceeds administrative limit — enhanced 5.2.3 flattened.
Updated Jul 1, 2026
SMTP 523 is shorthand for enhanced status 5.2.3, "message length exceeds administrative limit" (RFC 3463): the recipient server rejected your email because it is larger than the per-mailbox size limit it will accept. It is a permanent failure delivered with basic reply code 552. Fix it by cutting total message size: compress or link large attachments, trim inline images, and stay under the receiving server cap.
Reduce total message size or link large attachments.
What it means
"523" is how many clients and logs render enhanced status 5.2.3, defined by RFC 3463 section 3.3 as "message length exceeds administrative limit". The receiving server accepted the connection and recognized the recipient but rejected the message because it is larger than the size limit set for that mailbox. It is a permanent failure, so retrying the same message unchanged will not help. One accuracy note: 523 is not a real three-digit reply code. RFC 5321 section 4.2.3 defines no 52x reply. Over-size rejections use basic code 552 ("requested mail action aborted: exceeded storage allocation"), and the "523" you see is the dotted enhanced code 5.2.3 with its dots stripped. A full response usually reads "552 5.2.3 Message length exceeds administrative limit".
Common causes
Headers plus body plus all attachments, after MIME and base64 encoding, exceed the size limit on the recipient mailbox.
Base64 inflates attachments by about 33%, so a 15 MB file on disk becomes roughly 20 MB on the wire, and a "20 MB" attachment can blow a 25 MB cap.
A message your sending platform happily accepts can still bounce at the destination, which may enforce a smaller administrative limit.
5.3.4, "message too big for system" (RFC 3463 section 3.4), is a per-system rather than per-mailbox cap. Both surface as 552 and are fixed the same way.
How to fix it
- Measure the encoded size, not the file size
Multiply raw attachment bytes by about 1.33 (4/3) to estimate on-the-wire size, then compare against the recipient cap.
- Move large attachments out of the message
Upload to cloud storage such as S3 or Drive and send a download link. This is the most reliable fix and sidesteps every downstream size limit.
- Compress or downscale
Zip documents, and re-export images at screen resolution instead of embedding multi-megabyte originals.
- Split the content if it must be attached
Send it across several smaller emails when there is no way to reduce it.
- Check your provider cap
Common limits: Amazon SES 40 MB MIME-encoded via the SES v2 API or SMTP, SendGrid v3 Mail Send API 30 MB total message size, and many corporate Exchange or Microsoft 365 servers defaulting to 25 to 35 MB.
Paired code
A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:
Common questions
Is 523 a real SMTP reply code?
No. RFC 5321 defines no 52x reply code. "523" is a flattened form of enhanced status 5.2.3 ("message length exceeds administrative limit"), which is returned with basic reply code 552.
What is the difference between 5.2.3 and 5.3.4?
5.2.3 is a per-mailbox size limit set for one recipient. 5.3.4 is a per-system limit on the receiving server as a whole. Both come back as 552 and are fixed by shrinking or linking the content.
Why does my 20 MB attachment fail a 25 MB limit?
Base64 encoding adds about 33% to attachment size, so a file near 20 MB on disk lands closer to 26 MB on the wire, past a 25 MB cap. Size the message after encoding.