I found an other bug with french accents in version 3.1.2...
On the web interface in my account, in "Options->mail" if I check "Away
message Enabled:" and I put french accents in the "away message" I
enter, these accents are replaced by "?" marks in the email that people
receaves telling them I'm away.
As per Dan Karp from Zimbra, I added Bug 9051 in bugzilla.
Thanx all for this great product!