Problem not solved.
Cannot sent mail from this computer but can receive mail.
I used Mail preferences, advanced, and check-marked SSL, which also automatically changed the port to 995. Password is correct on Mail Preferences.
Network Utility shows 126.96.36.199 is a comcast mail server.
Mail has previously worked until last month.
Thanks, Neville Hillyer. Your advice was helpful, but in os 10.4, mail 2.1.3, and Comcast setup advice online there were some port differences that solved the problem. Thanks to NH I saw the SSL option was not consistent on outgoing versus incoming; saw two different sequences to use mail preferences for setting up incoming versus outgoing; and saw on COMCAST set-up that they like 2 different port numbers. My other newer Macs have not had this problem (yet) which seems strange since I saw port 110 in all my Macs.
a-- accounts Information tab type POP, my userid, mypassword, and on advanced tab incoming port should be 995, SSL checked, Authentication should be Password
b-- accounts information incoming server mail.comcast.net, my userid and mypassword, outging server smtp.comcast.net:myuserid, server settings port 465, SSL checked.
c-- in this old version mail 2.1.3, there is not save button until I select another account and a pop-up indicates my account changed and offered the save button which I selected. Mail then asked for my account's password; after I entered it, the outgoing test emails went out immediately.
Mail sending works again.