I've stumbled on this a couple of times.
Everyone's voice mail will forward just fine to their emails except 1.
One persons voice mail is not forwarding to email. The email address appears to be correct but logs show it rejected.
Finally, after exhausting just about everything I find the answer:
The email address has a space after it. So instead of being "email@myemail.com" it's "email@myemail.com ".
Very hard to see just looking at it. Easiest way to see it is to put your cursor on the email field and enter <ctl>a.
This will highlight everything in the field including the space.
Delete the space and everything works fine.
Just thought I'd share.
Ralph