FortiMail
FortiMail provides advanced, multi-layer protection against the full spectrum of email-borne threats
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Description
This document explains how to configure maximum email message size limit.
Scope
FortiMail
Solution
The maximum email size that the FortiMail can relay is configurable between 1 and 204,800 kilobytes. This option cannot be set to 0 (disabled).
The maximum size of the message is set in Profile > Session, under SMTP Limits (Cap message size (KB)).  Then this session profile needs to be applied to an IP Policy to take effect.
There is also domain setting for the message limit under Mail Settings > Domains
* For outgoing email, only the size limit in the session profile will be matched. If there is no session profile defined or no IP-based policy matched, the default size limit of 10 MB will be used. 
* For incoming email, the size limits in both the session profile and domain settings will be checked. If there is no session profile defined or no IP-based policy matched, the default size limit of 10 MB will be compared with the size limit in the domain settings. FortiMail will use the smaller size.