When user faces issues in email it due to domain MX pointing to remote server

in This case login to server and check domain entry whether in local or remote domains

grep domain.tld /etc/remotedomains

If domains name is not in /etc/remotedomains it will be in /etc/localhost

Change the domain MX record in zone file to fix this isssue

ONce MX records are changed run command /scripts/checkalldomainsmxs –yes

Now check the domain entries in grep /etc/remotedomains

The domain entry will be present in this file

Now remote emails should work fine