Exchange maximum send and receive message size not working
I see a lot of posts in the newsgroups about the maximum message size limits not working correctly. People are continually trying to change global message size limits, send connectors, etc... I keep meaning to write about this.
Out of the box, Exchange's maximum send and receive message size limit is 10MB. But, that is the "internal" message size limit. When a message is sent from one Exchange server to another within your organization, the format remains in MAPI format. This means that attachments remain in their original binary format.
However, if you send a message OUTSIDE of your Exchange organization the message must be converted in to an Internet standard. This standard is almost always MIME (Multipurpose Internet Mail Extensions). Binary attachments (ZIPs, DOCs, PPTs, XLSs, etc...) must be converted from there standard binary format to Base64. When an attachment is converted to Base64, the resulting attachment size is about 37% LARGER than the original file.
So, an internal e-mail message that is around 7.5MB (including attachments) will expand to just over 10MB. Exchange will issue an NDR that the message size is too large.
When explaining to your users the maximum send and receive (outbound and inbound) message sizes, it is best to explain that the "internal" limit is 10MB, but the external limit is around 7MB.