The message cannot be delivered (550 5.0.350 Too many MIME parts)

You receive a non-delivery report (NDR) that contains one or more of the below errors:

The receiving email server outside of Office 365 reported an error when your Office 365 account attempted to send a message to it.

Error Details
550 5.0.250 Remote server returned an error -> 550 Too many MIME parts: 180 > 150

Here, there are 180 MIME components in your message (email conversation), and 150 refers to the maximum number of MIME parts that the recipient's email server permits

Resolution:

Messages that use too many attachments or images are rejected by the recipient's mail server and create an NDR. Your mail server (SMTP server or EOP) generates this NDR when it cannot deliver your message to the recipient's mail server.

Try one of the following solutions to resolve this issue. It is likely related to the recipient's email server's security or policy settings.

  • Request the recipient's domain administrator for security and policy setting changes on their mail servers so that they can accept and allow messages with more MIME components.
  • Send email with fewer images and attachments, if you are replying in the middle of a long conversation thread.
  • Make sure that you don't include many pictures in your email signatures. If your signature contains many graphics, and you are taking part in a long email conversation, then your email may exceed the limit for MIME parts that a recipient's SMTP server can handle.

Few useful Tips:

You can reduce the number of images in email signatures by using Sigsync service:

  • While setting rules you can opt Use this signature in the first email and different in subsequent emails.
  • You can opt Apply signature to the first email only (don't add signatures to subsequent emails) in Preferences. 
  • You can select the option Apply signature when the first email is sent and remove it from the email conversation when subsequent email is sent in Preferences. 
  • You can select the option Apply signature to every email, but remove its previous occurrences in the email conversation in Preferences. 


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