Message Fragmentation and Reassembly
Section 22.214.171.124 of RFC 2046 (Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types) describes a feature called "Message Fragmentation and Reassembly" that lets you send a message in several parts and have them transparently reassembled back into the original message by the recipient. This allows a email client to improve performance when using a slow link (because the parts are sent in parallel) and to circumvent size restrictions. In Outlook Express this feature is selected using Tools -> Account -> Properties -> Advanced -> Break apart messages larger than __ KB.
Occasionally somebody asks in the forums if Thunderbird supports this feature. It doesn't, very few email clients do. Outlook and Outlook Express support sending messages using this feature. Supposedly all Microsoft email clients recombine the multiple parts by default when fetching new mail. Its not clear if any webmail implementations support it.
This functionality originally caused a problem because most scanners and filters were not able to search the multiple parts, making it an easy way to send viruses without detection. This should be less of a problem nowadays. All Symantec gateway products for example block multi-part MIME messages by default.