Knowledge Base

Your message is not delivered (550 5.0.350 Too many MIME parts)


Your message does not reach the recipient, and you receive a non-delivery report (NDR) with the following (or similar) error:

When [Office 365] tried to send your message, the receiving email server outside Office 365 reported an error.
Error Details
550 5.0.350 Remote server returned an error -> 550 Too many MIME parts: 150 > 100

where the first value/number (e.g. 150) is the number of MIME parts in your message (email conversation), and the second value (e.g. 100) is the number of MIME parts allowed by the recipient's email server.

If you send this message without an email signature added by CodeTwo software, the message is delivered normally.


This NDR is generated by your mail server (SMTP server or EOP) when it is not able to deliver your message to the recipient's mail server. The message is rejected by the recipient server because it contains too many attachments or images.

This issue is related to the security or policy settings on the recipient's email server. To solve it, try one of the solutions below.

  • Ask the admin from the recipient's domain to change the security and policy settings on their mail servers, so that these servers accept messages with more MIME parts.
  • Try sending messages that contain fewer images and attachments, especially if you are responding within long conversation threads.
  • Reduce the number of pictures in your email signatures. If your signature includes a lot of graphics, and you are taking part in a long email conversation, then your email may exceed the number of MIME parts accepted by the recipient's SMTP server.


Our software allows you to reduce the number of images in email signatures:

  • In the signature management app, you can add a different signature to your first email and different to next emails in a conversation. Learn more
  • When editing your signature template, you can set the program to add this signature to every email in a conversation but remove its previous occurrences from this conversation. Learn more
  • Or you can use the other available options, as explained in the user's manual.

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