Long delay in sending emails in Outlook 2007

[GeeJay]

I use Outlook 2007 for sending and receiving e-mails via a POP3 account with BT Yahoo. A strange problem occurred 2 days ago when I discovered that e-mails in my sent box were not actually arriving either to the addressee or into my POP3 account for about 5 or 6 hours. The problem still persists in spite of various repair and change procedures to my account setting. All account settings appear to be OK. I have no problem receiving e-mails and no problem sending and receiving direct from BT Yahoo. The problem only exists with Outlook.

Would be grateful for any advice or help