Cannot program two outgoing servers

A

AC Question

I have a Samsung S7 and an MTN contract and use Telkomsa wifi. Using outgoing server as smtp.dsl.telkomsa.net will allow sending email under WiFi, but not when away from WiFi. I then have to change the outgoing setting to mail.mtn.co.za and only then I can send emails in cell phone areas.
This seems ridiculous! I have been sent from MTN to Telkom to Samsung for three weeks. It would appear that the Samsung S7 does not have a feature to enter more than one outgoing server address. All other phone do -especially iPhone.
Is this possible?

Very frustrated
regards.
Mike Bridgeford