Trouble sending msgto specific recipient

Hi there,
We have one user that's trying to send an email to one specific recipient and that email just doesn't want to go through. We've checked that the email address is correct. We're using a POP3 account and don't have the mail server onsite. She's fine sending any other emails, but when she try to send email to this specific user it just get stuck in the outbox. That recipient's email is working fine, because we've send email from a couple of the other PC's to that address and that worked.

Please let me know if anyone knows why this is happening.

Many thanks.