email/domain issue

I have an internal pop3 mail server. One user cannot send one user an email. Undeliverable states that the user is invalid. Everyone else inside the domain can send to that person. That person can send to the person that can’t send to them and they will receive a reply. It is just an original email to the recipient that will not work. This has just started since we changed ISPs and network addresses. Nobody else is having this problem. I checked the spelling on the rejected email and the address is correct. any suggestions?