RE: Port forwarding confusion

I have a hard time getting the following situation sorted out
and would appreciate any input to solve it.
I'm in a firewalled LAN which provides a ssh-proxy to access
outside machines which I have to use like so:

1) build tunnel
ssh ProxyUser@SSHPROXY -L 2022:OUTSIDEBOX:22

2) make connection
ssh -X OutsideboxUser@localhost -p 2022

What I now wish to do is to access an outside mailserver
(especially port 993) through this proxy and am stuck how to
conceptually do this - build another tunnel through the tunnel?

If anybody can nudge me in the right direction, I would be
very grateful.

ssh ProxyUser@SSHPROXY -L 2022:OUTSIDEBOX:22 -L2993:OTHERBOX:993

Connecting to local port 2993 would connect you to your mail server while
keeping the existing tunnel for port 22.