I think I misread the first post and missed the 554 error - think I just read the "denied" and "blocked".

My guess is that this is an ATT broadband connection and he is trying to send the company mail through his company SMTP server, which is probably provided by the company web host, though there can be many causes of that error.

The first thing I would try is if you are using port 25 as the sending port try changing it to 26. ISP's don't necessarily like when you are attempting to contact another email server through their connection - it's a sign of spam especially on port 25. Another thing to look at, if the above doesn't work, is changing the company email account settings to use ATT's SMTP server for outgoing email. This eliminates the extra SMTP that might trigger some red flags.

In all cases you want to make sure that the client authenticates before sending mail - It's a setting in the client and is usually set to use the login user/password.

The best, but most complicated solution would be to use SSL for company email because the ISP can't see what's being transferred and won't know that mail is being sent through another SMTP server, but that really depends on whether the host provides SSL support for email (some do, many don't). An option here would be to implement a forward, or integrate the address, with Gmail which uses SSL for client access.

BTW, I checked the IP and domain and it does NOT appear on any of the major spam blacklists, which is a very good thing!

Cool! Keep us updated. I'm curious.