552 5.3.4: message headers too large
Exchange Online rejected the message because its headers add up to more than 256 KB, the limit for all header fields together.
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- Permanent (5xx)
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At a glance
- Code
- 552 5.3.4
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
552 5.3.4 Header size exceeds fixed maximum size.
What it means
Exchange Online limits the size of a message's headers: 256 KB for all header fields together, and 32 KB for any single header. The body and attachments do not count toward it. When the total is exceeded, the message is rejected with 552 5.3.4. Because headers are plain text, their size grows with the number of characters in each field and the number of fields, for example from long chains of Received lines, large custom headers added by an application, or very long lists of addresses in the To and Cc fields.
Common causes
- An application adds very large custom headers.
- The message passed through many systems that each added headers.
- Very long lists of addresses fill the To or Cc headers.
How to fix it
- 01Look at the source of a copy of the message to find which headers are largest.
- 02Remove or shorten custom headers added by the sending application.
- 03Send to large audiences through a distribution group or in batches instead of listing every address in To or Cc.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Exchange Online limitslearn.microsoft.com
Related codes
- 552Storage allocation exceeded.The receiving server aborted the message because it exceeded a storage limit, usually a full mailbox or a message larger than allowed.
- 5.3.4Message too big.The message is larger than the maximum size the receiving system accepts for any message, so it was refused and has to be made smaller.
5.3.4 at other providers
- 552 5.3.4Message too large.The message, with its attachments, is larger than Gmail accepts, so it was rejected and needs to be made smaller or sent another way.
- 552 5.3.4Too many attachments.Gmail rejected the message because it carries more attachments than Google allows in one message, whatever their size.
- 552 5.3.4Message headers too large.Gmail rejected the message because its header section as a whole is larger than Google allows, or holds too many header fields.
- 552 5.3.4A single header is too large.Gmail rejected the message because one of its header fields holds a value longer than Google allows for a single header.
- 552 5.3.4Header name too long.Gmail rejected the message because the name of one of its header fields is longer than Google allows.
- 552 5.3.4Subject line too long.Gmail rejected the message because its Subject header is longer than Google's header size limits allow.
Further reading
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