I had a look at the PC involved...
They are using Outlook to front end their google account. So they read their mail in from Google (POP3) and then obviously send new mail out (under their googlemail address).
NOW! What I noticed was, is they are using their ISP's SMTP server (newnet). I'm wondering if some places (eg: AOL) are funny about this in someway? ie: They see an email with a from address of googlemail, but having come from somewhere else?
Is that possible?
I tried to change the SMTP server to google but just could not get it to work so I'll contact NewNet about that...