If mail is not going, check MX and see that the domain MX is hosted external. Depending on that add the domain in the below files.
/etc/localdomains: Contain a list of domains that have email hosted locally
/etc/remotedomains: Contain a list of domains that have email hosted remotely
/etc/localdomains: Contain a list of domains that have email hosted locally
/etc/remotedomains: Contain a list of domains that have email hosted remotely