Message size increases for sent emails in Outlook

At times, you may have to send out large attachments to an external user and you are aware about their maximum size limit. You send out the attachments considering the size limits at recipient end. But to your surprise, the email sent out from your Outlook appears to be of more size than what you have actually sent and you receive a Non-Delivery report AKA NDR stating that your email size violates the email file size limit of recipient server. There is a reason as to why message size increases for sent emails in Outlook.

This is because additional 33% translation encoding increase the size of the email sent out from your tenant to other tenant. Message size increases due to MIME conversions, internet header inclusions etc. This additional increase in size does not happen when the emails are sent internally. You can refer the technet article for more information on Exchange Limits.

Key points

  • Office 365 Administrators can specify a custom limit between 1 MB and 150 MB. However, the size of message you can send or receive also depends on what your email client or solution supports.
  • You can send and receive up to 150 MB messages between Office 365 users (where the message never leaves the Office 365 datacenters). Messages that are routed outside of the Office 365 datacenters are subject to an additional 33% translation encoding increase, in which case the maximum message size is 112 MB.
  • OWA accounts for the possibility that your message may be subject to the 33% encoding increase and restricts the size of message you can send to 25% less than the configured setting. For example, if you customize your settings for a 100 MB maximum message size, you can send messages no larger than 75 MB.
  • You cannot attach a single file that exceeds 35 MB. Additionally, you cannot attach files that collectively exceed 35MB. For example, if you attach one 34MB file, you can only attach an additional 1 MB file.

Additionally, you can check your tenant’s Message size limit in Exchange Admin Center.

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