By default sipxcom/uniteme considers itself authoritative for whatever the sip/network domain you entered during installation. If the sip domain is the same as the network domain, and you're using an external MTA rather than sipx/uniteme, then you'll need a custom MX record to point to that external MTA. You'll likely also need to ensure the MTA is allowing delivery from sipx/uniteme.
First check if there's an existing outside DNS MX record for that domain:
dig -t mx example.domain.here @
8.8.8.8
If there's an answer pointing to an external MTA, but you don't have the same MX record defined within the local sipx/uniteme zone (/var/named/default.view.example.domain.here.zone), then you'll likely need to add a custom DNS record for the MX (system - services - dns - custom records) to add that external MTA it into the local sipx/uniteme dns zone. Paste the answer part as your record contents, see attached example. Do not edit the zone file directly, as it is managed by sipxsupervisor (cfengine) by default so any direct edits will be overwritten.
The relevant log for email delivery is /var/log/maillog. Every delivery attempt would be noted there, as well as the error it encountered if any.
Regards,
Matt