If I could get some advice on this matter I'm working on, I'd GREATLY appreciate it.

I keep running into dead ends on this one, it's pretty odd, mainly because there's nothing apperently consistent about it, so I have no real leads to follow. This client of ours is on a laptop that goes back and forth between office, home, trips, etc. (he's also a VIP client -- ya know, potentially impatient big shot -- so I'm trying to figure this out as fast as possible). Basically, sometimes he can send email, sometimes he can't, and he can always receive email without any problems. Here's the breakdown:

1) He's using Outlook (I forget exactly which -- 2000 or 2003).
2) POP account. POP3 and SMTP connections are properly configured to the right servers. User/pass info is correct.
3) He can always receive email. The problem only occurs with sending email.
4) Sometimes he can send with no problems, others he is prompted for user authentication, but then the authentication window just appears over and over again instead of allowing him to send the message.
5) He also gets "No transport provider was available for delivery to this recipient." bounce back messages.
6) The problem shows no pattern -- Sometimes he can send mail, sometimes he can't -- happens at various different locations, on both ethernet and wireless. So we can't narrow it down to happening at location A or B, or on wireless or off, as it works sometimes in all scenarios, then doesn't others.
7) His mail provider responded that there is no problem on their end, so it has to be on the user's end.
8) Both POP3 and SMTP go through the same server with the same authentication.

So I'm in a bit of a quandry here. Why does it only authenticate part of the time (I have yet to reproduce the problem when I've been on his machine)? And why will it let him receive mail even though he can't send? This has evidently been going on for a while now, so it's definitely not a problem that's going to resolve itself anytime soon.

Any thoughts?