Intermittent problem sending mail from Outlook 2007

I occassionally get the email below after attempting to send an email. It often appears exactly as below, which is to say it doesn’t list any recipients. Sometimes it does. I do not use Microsoft Exchange. I am using Outlook to retrieve POP mail and send through my ISP’s SMTP server. I have the account configured to require login to send mail, and to login to incoming mail server before sending mail (although I never understood what that option is really all about).

I cannot create conditions to force it to occur, although it occurs at least once a week.

Any ideas what this means, if there is really a problem, and how to fix? I’ve combed though Microsoft forums but generally either people are using Exchange and the fix is irrelevant, or recommendations are to adjust my settings to be what I already have.

From: System Administrator
Sent: Thursday, December 09, 2010 9:37 AM
To: CookingWithGas
Subject: Undeliverable: Team Status Meetings

Your message did not reach some or all of the intended recipients.

Subject: FW: Team Status Meetings
Sent: 12/9/2010 9:37 AM

The following recipient(s) cannot be reached: