Re: Been reading docs and I'm not clear
This is done from Home or where you have access, Setup your VPS server (System B) to listen with SSH on 8443 (if the allowed outbound from the Corporate firewall). You should use a second "Port" statement, not removing the other ones.
Port 8443
And then try without leaving *THAT* ssh session. Open a NEW terminal
ssh -p8443 -lyouruser yourhost.yourvps.com
Ensure it is good. Fix if needed.
Here is a config example for your Desktop (System A.)
This config setups the redirects pointing at your hosted mail server (System C) through you VPS server (system B)
greg@omg:~ [0] $ cat ~/.ssh/config
Host redirects
Hostname yourhost.yourvps.com
Port 8443
#Outgoing mail delivery sstmp/esmtps
LocalForward 1465 hosted.mailserver.com:465
#Inbound mail pickup (IMAPS)
LocalForward 1993 hosted.mailserver.com:993
# Inbound imap, not needed is using imaps
LocalForward 1143 hosted.mailserver.com:143
Then just point your mail client at "localhost" or 127.0.0.1 with the proper port for the proper service and it is done.