If I send a thread to a user and if the user's email address is hidden I get the following email message in my inbox.
But if I make the email address unhidden, then it works correctly.
Can you please remove the users from the pick list whose email address is hidden?
Your message did not reach some or all of the intended recipients.
Subject: Testing 1234
Sent: 10/14/2007 4:11 PM
The following recipient(s) could not be reached:
firstname.lastname@example.org on 10/14/2007 4:11 PM
The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address.
< mail.rackspace.com #5.1.1 SMTP; 550-Mailbox unknown. Either there is no mailbox associated with this>