N. Miller said:
The error is pretty explicit, and you aren't saying: Did you configure
Outlook to authenticate to the SMTP server? That error is saying that you
have not done that; but the server wants your UserID+Password.
Are you *on* Comcast's domain (i.e., on-domain) when you attempt to send
e-mail to their SMTP mail host? Or are you on someone else's network
(off-domain) when you try to use Comcast's SMTP mail host? E-mail
providers and ISP's usually don't care if you retrieve your e-mails from
off-domain but they definitely care if you attempt to send e-mails
through their domain while you are off-domain. When off-domain, they
don't know that you are authorized to use their resources, and they
don't want spammers hiding behind their domain (i.e., the spammer is
somewhere else but abusing Comcast's resources to spew their garbage).
Have you yet tried configuring the e-mail account that you defined in
Outlook so that Outlook authenticates to the SMTP mail host? If you did
but it is set to reuse the POP login credentials, select to specify
separate login credentials (and just reenter the ones you use for their
POP mail host).
Also, like many ISPs trying to thwart customer hosts that are infected
with a mailer trojan that typically spew out through port 25, Comcast is
changing to port 587 (for non-SSL connects; for SSL, use port 465). So
try changing to port 587 for SMTP (outbound). Comcast will also block
traffic over port 25 from their customers that they determine have been
violating anti-spam or anti-abuse quotas for *personal* e-mail accounts.
In that case, try port 587 (or 465 with SSL). If it still fails,
contact Comcast to ask why they terminated your sending privileges.