Non-delivery report 552 5.3.4 Header size exceeds fixed maximum size

If you receive the following non-delivery report when attempting to send an email, the message headers exceed Exchange limits.

Exchange applies strict message limits to the maximum size of all header fields in a message; this current limit is 256KB with individual headers limited to 32KB.

You can read more about this limitation from Microsoft below.

Exchange Online message limits

Emails sent that exceed this limit cannot be sent and will receive this NDR.

To resolve this issue, you must reduce the header size (combined or individual) of messages that are being sent by your users by addressing the underlying cause from the sent item.

This issue by definition cannot be resolved by Symprex, however our support team will assist where reasonable and possible to help your teams identify the root cause.