Delivery to the following recipients is still underway

This has just started popping up.
WinXP and OE 6
At first I thought it was a problem with a recipient. She had been having
connect problems.

Then my wife, who has a seperate account with the same ISP as me, had about
a dozen outgoing emails that got the same result.
Her account is on the same PC but is otherwise independent of mine.

As a quick test, I sent an email to the wife's address and it went through
immediately (at least as fast as I could switch accounts to see)

The entire message reads as follows:

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients is still underway after 7.2 hour(s):

* xxxxxx@xxxxxxxxxxx

Will keep trying and contact you if the message can't be delivered


The message also indicates an attachment "ATT00XX.dat" where XX represent a
decimal number.

My antivirus applications did not find anything amiss.

Where does this error arise and what should be done?