I am working on this issue as I have been for the past couple of days. I am not sure what is causing the problem but I can see that some mail servers are rejecting the e-mails. I think I have found the problem tonight but the best I can do is give it some time and see if we stop getting rejection replies.
Basically, the e-mails are not going out with the mousebits.com domain as they should, but rather a LAN server name. Some mail servers look up domain names to make sure they're not spam mail, and since this funny LAN URL is not registered they are rejecting the e-mails. As far as I know, the mail service with the website is working fine, the issue is with other mail services rejecting the e-mail.