This is happening currently to me. It is my believe that this is due to server limitations on the mail routes it allows (as some servers will only allow mailing to other addresses with the same name, if sending from ....*)
* Example:
x@x.com would only be allowed to send to a - z @x.com or other domains within the network under the server, however, sending the message from @hotmail.com or any address to any address works fine.. This is not a vbulletin problem, but one you should take up with your server administrator.