This solution applies to all Email Security Gateways, all firmware versions.
When an attachment is sent in an email, it needs to be encoded to ensure that it only contains valid characters. (Email servers don't handle arbitrary backspace, delete and null characters in the data stream well)
During the delivery process, encoding will increase the size of attachments by approximately a third, depending on file type, and the destination email server will un-encode the email when it arrives. This returns the attachment to the original size.
However, this can cause some messages to be rejected for exceeding the size limitation of the destination server, even when the original attachment does not exceed the threshold when not encoded.
Link to this page: