This is one article in the larger Troubleshooting SharePoint Alerts guide

Problem

Task lists can send an email notification when a task is assigned or changed (Settings > Advanced Settings > E-Mail Notification).

If you modify the Assigned To column of the task list to “Allow multiple selections” then the email may be sent to random, incorrect recipients – some names will be crossed out (strike-through) in the email.

References

• Random emails getting sent out when changing “Assigned to” to allow multiple items

• Email Notifications and Alerts problem

To Resolve

This is a known issue and was fixed in Windows SharepPoint Services 3 Hotfix (KB948945 21st February 2008)

“On a SharePoint Server 2007 site, you modify a task in a task list. When you do this, users other than the users who are assigned to the task receive the notification e-mail message for the change.”

This fix is believed to have been included in

• WSS3 SP2 (April 24th 2009)