I'm not sure exactly what client and what server you're trying to get to talk to each other, but you can't connect to the internal address of a computer that's behind a router from outside the router. The internal address doesn't exist on the outside of the router, any more than the external address (mail.companyname.com) exists inside (although a smart router knows that mail.companyname.com refers to it, so it may just turn the connection back inside without looking anything up - that's called a loopback).

As far as a certificate goes, that depends on the particular software - you don't need any certificate to access the inside of a LAN from outside its router, unless the particular software requires one.