Hi, L3OUGH,
As a relative newby to iMac myself … . You can get a list of all previous recipients as follows: In the Mail program, pull down the 'Window' menu and select 'Previous Recipients'. That gives you a small new window with a list of all addresses that you have written to. I haven't yet worked out how to copy/paste that to something you could use, but there is, in that new window, the option to 'Add to Contacts', so you could select all (cmd-A), 'Add to Contacts', then open Contacts and Print (select 'Style - Lists'). I haven't tried it myself, so - good luck!
