It is posible to forward your mail to an other address but leave a copy of the original message in your mail box.
Just make a .forward file in your home directory, containing something like:
otheraddress@...... ( mail is forwarded to the other address)
or
userid, "otheraddress@.........." ( a copy is left in your mail box )
or
userid, "other1@...","other2@.....",etz