message-alternative-emails

message-alternative-emails is a variable defined in `message.el'.
Its value is

nil

Documentation:
*Regexp matching alternative email addresses.
The first address in the To, Cc or From headers of the original
article matching this variable is used as the From field of
outgoing messages.

This variable has precedence over posting styles and anything that runs
off `message-setup-hook'.

You can customize this variable.