Ok, this one really is getting to me.
facts:
-customer uses Outlook express 5 with Win98 SE
-customer set up pop and smtp the right way. (she send me screenshots)
- on server end everything is also set up right.
- she can receive mail, but when trying to send she gets:

The connection to the server has failed. Account name, server name, Protocol SMTP, Port 25, Secure (SSL):no Socket error 10051, Error number: 0x800ccc0e

I've set up her account in my own outlook and it works just fine.
I've had her hit the send/receive button before composing/sending a new mail: no go.

Suggestions?

We had this same problem not to long ago. First of all make sure you have POP Before SMTP Relaying ticked in the Email Server Parameters. If you do and the user still has the problem, have them hit Send & Recieve a couple of times (not just once) and that should fix it. At least thats how we did it. I hope that helps :)

Ask her what her ISP is. I have a feeling it is one that blocks Port 25 mail relaying. If this is the case, she'll have to use her ISP's SMTP and just change the 'reply to' address, though I've heard that even *that* can block the email from going through.