[Spivak]

If you want your email to look the same on all clients text/plain is the way to go.

[Flimm]

That's not true. Plain text is usually hard wrapped with newlines, which leads to awful experiences on small screens.

[mi100hael]

You can send plain text encoding with format=flowed to avoid the hard wrap. Personally I prefer 80 chars, though. Still looks fine in landscape mode on any phone.