Let’s move discussion from Smarter message size limit - #4 by link2xt here.
While RFC RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types for fragmented mails exists, it is not widely supported e.g. by Web mail clients and Thunderbird.
One option to stay somewhat compatible would be to use split Zip files instead. Reconstructing Zip files from segments is better known than fragmented email.
For users with bad connection who cannot download large files, IMAP allows to request parts of the file with FETCH command requesting BODY[]<0.2048> to get the first 2 kB, then BODY[]<2049.4096> to get the next 4 kB and so on. This will allow to download large files without starting from scratch even if the connection drops in between. This would not help for message sending though and cannot bypass the mail server file size limitation, but may be worth implementing in any case as everyone sometimes receives large attachments that are difficult to download over a bad Wi-Fi connection in one go.