What are the MX Outbound message size limits?
What is a max size of a email message?


MX outbound message size limit is set to 26Mb (MIME encode).

For more information on message size limits, minimums and restrictions please refer to SMTP RFC (RFC 2821),

What is MIME encoding?

MIME (Multi-purpose Internet Mail Extension) encoding is in fact a Base64 encoding of a message.  The message is encoded when there is a need to send (email) binary data (attachments, images, etc) over a media that was designed to deal with text based data.  In this case an email.

When MIME encoding the "real" message size will increase due to the Base64 encoding by a factor of 33%.  So a max message size of 26Mb MIME Encoded is the equivalent of a real message size of 20.1Mb.  However please take into consideration when calculating the total Message Size it includes the Header Information, the Message Body and attachments.


Please note the message size has been set to the above and complies with the SMTP RFC standard. It's pointless making the limit bigger because there is no guarantee the receiving server has the same or larger size restriction set. In fact by default most MS Exchange servers have a message size limit of 10Mb so anything bigger sent to those servers would simply generate a postmaster failure.

If you need to send emails with attachments larger than the size restriction the its recommended you us an alternative to emailing and instead use a “mail big file” service where the attachments are uploaded to a secure server and a short email is send out with just the link to download the large attachment.