Masquerading in userspace [SOLVED - not via masquerading]

[Hamlet]

Edit: I clarified the details of the constraints. Changes are in italic.


I am facing the following need: I am administrator of my laptop (C) and I need to access services from some servers (S), let's say an IMAP server for example. While anybody can connect to the servers from anywhere (e.g. IMAP server is MicroSoft's), the servers serve only the connections which appear to come from selected network N (presumably by checking the source IP), and C is not in that network. But I can access a server G, which is in network N, as unprivileged user in some secure way (say, SSH).

How can I access the full services on S from C? that is, how can I set up C and run the proper userspace software on G so that I can access the IMAP server with my laptop e-mail client?

I have read a bit about VPN and IP masquerading, but I don't really know what to do with that yet.

Thank you!

[Phoenix591]

SSH port forwarding fits the bill pretty well. Heres a quick guide I found, looks like the part you'll be interested in is the "local" port forwarding.

TLDR:

[Hamlet]

Thank you for the answer, and I apologise for wasting your kindness. I realise now that there was essential information missing in the original message.
In my understanding, the servers are networking-wise visible from everywhere, but they allow only log ins from the configured networks.
For example, if I say I am "Hamlet", it acknowledges my request only if I connect from Elsinor because that's how its administrator has set it up; but if I say I am Fortinbras, then will maybe start a two-step authentication or check that mine is a Norwegian IP.

My understanding of tunnelling via SSH is that my original IP is not changed, and the tunnelling only allows my packets to actually reach the server. So if I connect from Witttenberg, my packets will pass through Elsinor's tunnel to reach the server instead of going directly, but they will still say I am connecting from Wittenberg (therefore causing the denial of the requested service). I may be wrong though.


P.S. I updated the original post so that other readers can get the right information at once.

[Hu]

SSH has multiple tunneling modes. The simplest, port forwarding, causes the ultimate destination to believe the client address is that of the sshd intermediary, not the true client. Please try it and report back.

[Hamlet]

And, guess what? you are right.
So for the IMAP access I used something along the line of

[Hu]