The 30 MB message size limit we advertise is the actual total size of the entire message
during transmission. Due to the encoding that’s required to transmit attachments via Internet e-mail, the size of an attachment as it is being sent through the e-mail system can be up to 150% of its original size. This is because the e-mail system must add internal headers and footers for each attachment and each attachment must be converted from an 8-bit format to a 7-bit format. This, unfortunately, is required by the underlying e-mail protocols that were designed for the transmission of text only, not for binary files. Internally within our Exchange e-mail system, messages are transported in 8-bit binary format, so that conversion isn’t required and the 30 MB limit is fairly literal. But messages to external recipients must be converted to the 7-bit format that is the standard for Internet e-mail transport (SMTP); this results in a de facto size limit of 20-22 MB per message.
Another possible issue is that the recipient's e-mail system has a lower size limit than ours. For example, if your message is 20 MB and the recipient's e-mail system has a 15 MB size limit, your message will make it through our system but will be rejected by the recipient's e-mail system.