microsoft outlook - email recall

hello gang..
quick question, this is the scenario.
one of the VP's secretary sent an email to the wrong group yesterday afternoon, nothing bad, just information email. She did a "recall" and some of them failed to be recalled.
i understand that once some one opens an email before being recalled, the recall will fail. But, she had a few people that just opened the email today in the morning and were able to read the mail, and it was not recalled yesterday afternoon. Are there permissions set on the exchange server that gives users or group permissions that their email cant be recalled ?
any ideas ?

thanks
Peligroso

I believe the recall can only happen if the email is in the user inbox and the users computer is logged on....
(Note: This method will not work under certain conditions, such as if the message has already been read or moved, if the message was sent in something other than Rich Text, if the recipient is not logged on to the Exchange server, or if the recipient is not using Outlook.)

Yes, they do have to be logged on and the email unread.

got all the answers i needed.. thanks guys.. you rock, if you want to take the time, you can stop by to answer my other post screen saver