Thanks, but which encoding is/are the most common preferred?
it's not a matter of which one is prefered

It's a matter of using the same one the mail was sent with.
Entoruage should auto-detect the encoding, but some e-mail clients are
notoriously bad at properly formatting the headers and make it hard for
Entourage to properly guess.
Also, when I send some emails, but not all, I get the following
message:"that message must be sent as a Unicode message. ... Do youstill
want to send it?"
Why do I get this message?
Well it means that you used some characters that are not part of the
"regular" set. Either accented letters for some East European
languages, other alphabets altogether or simply curly quotes, ellipsis,
etc.
In order to properly encode them, Entourage needs to send your message
with an encoding capable of handling then (Unicode).
Most clients can read Unicode nowadays so it shouldn't be a problem.
Corentin