XMIME is a command that is used to force the application/html MIME type 
for text or html documents only.

This command is present for those e-mail readers that pretend to be a
browser in that they try to show HTML attachments directly.

Example:
XMIME
http://www.ictp.trieste.it

The above example would send a reply with an attachment of type 
application/html. Most mail readers would offer to save the file 
after which you can use a regular web browser to view the file.

SEE ALSO

help mime