Send Email error message

[Fletch]

I have 2 seperate email accounts. During the send-recive process, I receive
this error message for one of the accounts:
Task 'mail.comcast.net - Sending' reported error (0x800CCC78) : 'Unable to
send the message. Please verify the e-mail address in your account
properties. The server responded: 530 Authentication required'.

I've removed the old address and re-established the account several times
and even though the information is correct the message persist. I've contact
the ISP and the address is correct and active.

Any suggestions??