That file may contain, among mail configuration commands, aliases for mail to use. An example of such an alias could be:
claude firstname.lastname@example.org +cantinIf mail is sent to group, it will then automatically be sent to both claude and wayne, which actually means email@example.com, the mail folder cantin, firstname.lastname@example.org and the mail folder wayne.
wayne email@example.com +wayne
group claude stan
For more information on the .mailrc file, see the Special Files chapter of Book II: Advanced Introduction.