[FIX] ir.mail.server: improve support for user names with emails

The previous fix in revision 5072 only allowed user names
that contained the exact same emails, but users will do
the wildest things like having `someone@domain.com` as
name but setting their email to `someone@domain2.com`.

This was blocked by our sanity check looking for a single
email in the From header. As this check is only done
in order to provide a better error message, it should
not impact valid cases.
Modifying the check to pass when at least one email
was found should be enough to catch most invalid cases,
without requiring a more advanced analysis of the
header value (the RFCs allows very strange things!)