Windows 7 Ultimate won't send email sometimes

I need some ideas about what I'm missing with this issue.

We have a network of 50+ users in the office. I've configured all of them and all their replacement computers, so I've done this setup at least 100 times.

Our website and email are hosted by an outside company. We are configured with POP3/SMTP accounts in Outlook.
Our in office internet connection is 2 bonded T1's with an unrelated ISP.

A few of our users have laptops that they dock in the office as workstations and then take home. They communicate with Australia and Asia at all hours because of the time difference.

One user has Windows 7 Ultimate and Outlook 2007 and has me stumped.

This user has not been able to send email via Outlook since day one under certain circumstances. He can always receive.

What I've seen so far:
- The only successful send outside the office has been via mobile broadband.
- It doesn't matter if he's at McDonalds, at home, at a hotel in Moscow, he can't get outlook to send emails outside the office (except for that mobile broadband device).
- I thought it might be a wireless security issue but one hotel he was connected by ethernet and at home he tried an ethernet connection without success.
- I've put him on the office wireless and it works fine.
- All 4 other laptops can send fine from anywhere. None are on Ultimate, they are either Windows 7 Pro or XP.
- User's previous XP laptop never had an issue with the same configuration, it's not an issue in his email account at the host.
- Blackberry on the same email account sends and receives fine, but not practical for his lengthier emails.

So what am I missing? What obscure little annoyance is in Win 7 ultimate that is messing with me? I can't think of where to look anymore and searches aren't yielding anything useful.