Please look at the undeliverable email you are getting as it is likely that the mail server you are using, via your email provider to send emails to us at xxxx@mail.crmdomain.com is black listed.
This is something you can check by taking your client host IP address and checking this on a blacklist site called www.mxtoolbox.com where you can go to the Blacklist option and enter your client host IP address to check to see if it shows up on any email blacklists.
If so you will need to contact your email provider directly to let them know that they are on a blacklist and ask them to resolve this by getting themselves removed.