Knowledgebase: Inbound Mail Flow
'552 5.3.4 Message size exceeds fixed maximum message’ even though Email size is less than max receive size
Posted by Muhammad Ahmed, Last modified by David Szoke on 05 January 2018 08:56 PM

Email size showing up in ProofPoint Essentials basically reflects overall size of the Email (including attachment)

Emails get MIME encoded . The binary files are encoded to plain text and decoded on the recipient side.

The result of this process is overhead in terms of file sizes.

In general, MIME encoded binary attachments are approximately 137% of the size of the original, pre-encoded file.

For the current file size, encoding will take this, and add about 37% of the original file size

For example, if you attach a 3MB .zip file to an email, the file will exceed 4.1MB when it is in transit between servers.

When setting up email size restrictions on the server or in the Essentials custom filters, the MIME encoding process should be taken into consideration.

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