An email message may be refused delivery with it being oversized as a reason when the attachment appears to be under the size limits. In this situation, the following error message is usually returned:
552 5.2.3 Message Size Exceeds Fixed Maximum Message Size
In order for a binary attachment to be transmitted via SMTP, it must first be encoded for transport. Typically, within a MIME message, the attachment is Base64 (b64) encoded for transmission by the mail client. One of the side-effects of the b64-encoding is 20-30% overhead. As a result, the encoded attachment size will increase causing disk size to grow. This can be confusing when email policy is configured to allow attachments below a certain size.
For example, a PowerPoint presentation that is 9MB on disk, will be approximately 11MB once converted from its original binary format into a b64 string. An email policy that is configured to strip attachments over 10MB will strip this attachment while sender and receiver still believe that the attachment was under the specified limit.
Another point worth clarifying is that typically the message size rules take into account not only the attachment but headers and message body as well. Simply put, the entirety of the message, in its encoded format, is going to be taken into account by the recipient's server and its message size threshold limitations.
At the time of this publication, iSheriff has b64 encoded message size limit set to 100MB in order to ensure uninterrupted email relaying. While we do not recommend sending such large emails, our infrastructure is certainly configured to accommodate. If you encounter a situation where a message is being refused due to its size, most likely the iSheriff infrastructure accepted the message without an incident but was denied when trying to relay to the recipient's SMTP Server.
Please contact iSheriff Technical Support Team for further assistance.