ok, reviewing your first post:
You can send and receive to/from
You cannot send or receive your personal email. You have it setup at the ATT BIS website. You have received the service books for that email address?
Look at Options > Advanced > Service Books. You should see a Service Book entry for each email address you have services for, noted by the "@" symbol in the line. You will see one for the "@att.blackberry.net" address.
Do you see a line for the "@73dotcom" address? If not, then that account is NOT correctly setup. You can recheck the setup at the ATT BIS site, and always can resend the service books.
Where can I log into my BIS account? - BlackBerryFAQ
How do I resend my service books? - BlackBerryFAQ