Message is a
IDNA-compliant posting agent. The user
generally doesn't have to do anything to make the
IDNA happen—Message will encode
non-ASCII domain names in From,
To, and Cc headers automatically.
Until IDNA becomes more well known, Message queries you whether IDNA encoding of the domain name really should occur. Some users might not be aware that domain names can contain non-ASCII now, so this gives them a safety net if they accidently typed a non-ASCII domain name.
The
message-use-idna variable control whether
IDNA is used. If the variable is
nil no IDNA encoding will ever
happen, if it is set to the symbol ask the user will
be queried, and if set to t (which is the default if
IDNA is fully available)
IDNA encoding happens automatically.
If you want to experiment with the IDNA encoding, you can invoke M-x message-idna-to-ascii-rhs RET in the message buffer to have the non-ASCII domain names encoded while you edit the message.
Note that you must have GNU Libidn installed in order to use this functionality.