Configure mail Alert

[Michael Steele_2]

what is your ip address? are you on different subnets to the mail server?

[Michael Steele_2]

This is a DMZ or firewall issue I bet.

[Ralph Grothe]

Probably you cannot reach the mail exchanger for domain mtn.com.af. directly.
But you hand it over to your smarthost you configured anyway.
I assume that your smarthost is rejecting it because it hasn't got a proper return address that it can advertise (in the envelope) to the mail's recipient in the Internet (for bounces or replies).

What does your intranet DNS resolve from your hpux host as mail exchangers for the mail's recipient domain?

e.g.
$ nslookup -q=mx mtn.com.af.

You could also try what your hpux host's sendmail makes of this

$ /usr/sbin/sendmail -bv billing-support@mtn.com.af

Anyway, I would contact your smarthost's postmaster and ask what needs to be configured and what the prerequisites are.