When Microsoft Exchange sends an email message to another host via SMTP, the message size may change due to the encoding used to package the message. This is true also for messages with attachments, because the only way to send email attachments is to convert them from plain ASCII to MIME or UU-encode the message.

Even if an attachment is less than the size limits prescribed by Exchange, the attachment may not be accepted because its MIME-encoded or UU-encoded size is too large.

This happens most often when administrators set limits for inbound SMTP mail. An incoming MIME-encoded e-mail with attachments can increase in size anywhere from 30 percent to 40 percent, depending on how many separate attachments, line breaks, MIME headers or other non-data elements are in the message.

The exact size can vary enormously, especially since mail systems all behave a little differently when converting e-mail and attachments to MIME. The same problem exists in reverse, where messages sent from your domain will be constrained by message limit sizes on other hosts. Likewise, mail sent from your domain is going to expand anywhere from 30% to 40% in size when converted.

A third-party application, such as UUDeview, can help you find out how much a MIME or UU-encoded version of a given file will increase. (Note that this tool does not calculate things like message size overhead, but it can still be helpful.)

The exact maximum incoming and outgoing message size is going to be up to the email administrator, but should be set with these caveats in mind.