First of all--no, any kind of music file can be used. A MIDI file is MUCH
smaller in size than an audio file though. If you have an MP3 file embedded
in the e-mail, and the recipient has a dial-up connection, they might not be
too happy with you waiting a long time for your e-mail to download.

To create a MIDI file--you need to have MIDI software, and you have to have
some musical knowledge. You might want to look at the newsgroups
alt.music.midi and comp.music.midi. However, there are LOTS of MIDI files
available to download for free from the Internet, so if you are looking for
a particular song, you might want to just type the name of the song you are
looking for, and MIDI into a search engine.

I occasionally receive an email that contains a music background that plays
when the message is opened. Is a midi file the only file format to use to
create a musical background? If so, how does one go about creating a midi
file?

My thanks,