Variable: rmail-user-mail-address-regexp

Regexp matching user mail addresses.
If non-nil, this variable is used to identify the correspondent
when receiving new mail. If it matches the address of the sender,
the recipient is taken as correspondent of a mail.
If nil (default value), your `user-login-name' and `user-mail-address'
are used to exclude yourself as correspondent.

Usually you don't have to set this variable, except if you collect mails
sent by you under different user names.
Then it should be a regexp matching your mail addresses.

Setting this variable has an effect only before reading a mail.