Mail relay server behind firewall

Hello everybody.

First of all, I would like to apologize if this question is stupid but I feel like I am missing something…

For more than 10 years, our small company uses the mail service of cPanel on a VPS. The clients (Thunderbird on Ubuntu) retrieve the emails via POP and automatically delete them from the server after 25 days (so emails are kept on the client side only and the server never gets full). We have some shared email addresses (such as “”) so we often ask each other “did you reply to this email?”. I am thinking about switching to IMAP on MIAB to solve this problem.

I understand I would have to convert our mailboxes to IMAP and upload them to the new server but I don’t like the idea of storing 10 years of emails (~500GB) on a machine connected to the internet. So I was wondering if I could use a kind of “mail relay” between the clients and MIAB.

The idea is:

  • The “mail relay” would be in our LAN (behind our firewall) and would act as a client of MIAB to retrieve (via POP) and send (via SMTP) all emails

  • The Thunderbird clients (in the same LAN) would connect to the “mail relay” to retrieve (via IMAP) and send (via SMTP) all emails

I found this project but it looks quite old.

So to summarize, I would like to have emails stored on a machine within our LAN + offer the possibility to share an email address.

What would you advise?

Thanks in advance for any help!

I thought this setup would be more popular but I didn’t get any answer. Can someone kindly explain me how I could set up a “relay” (within our LAN) to act between our Thunderbird clients (in the same LAN) and our MIAB server (on a VPS outside our LAN) ?
Thanks for any help!