your outgoing mail server is configured wrong.
go to mail>prefs>accts>acct information and set your SMTP properly.
everytime i log into mail at a different access point that uses a different mail server, i have to set up a new SMTP
examples:
mail-hub.optonline.net
mail.bellsouth.net
it happens often when i use somebody's cable modem (wired or wireless)
and i find i always have to delete the ones i'm not using mail sometimes doesn't pick the correct default. delete the incorrect ones then quit mail and restart again.
for some reason mail has a problem using the default setting you indicate and invariably picks the wrong one not giving you the choice to re-send using the correct SMTP
that should fix it
