If your getting not a valid address, that's a clue that part of the problem
is the address you are typing. Try copying and pasting the address into
address book from whatever source you have for the address. Once in the
address book, open a message box to type a new message, click To, that will
open a list of addresses in the address book, then just double click each
address to which you wish to send the message. That should place them in
the address block in the proper format and resolve any problem caused by
perhaps your not knowing how to properly type them into the box.
Just a note, many people do not like to receive messages with their address
listed so you might want to use the BCC function which will hide all but the
first address which should go into the regular address box.