Is it always the same addresses that time out?
Email messages get to their destination by bouncing from server to server, if the journey takes more than a certain amount of steps, or the incoming server refuses the message more than 3 or 4 times the message will be bounced back to the sender.
There could be a number of reasons for the receiving server to reject the email, it could be that the email address doesn't exist, so check you have the correct address.
Or it could be that the recipients ISP is actively blocking emails from your ISP, perhaps due to spam or virus attacks from someone using your ISP.
Or perhaps the recipients mailbox is full.