Storing mail only on the server

Question

Email

Actually there is a small enterprise on 50 computers. Everyone has their own, corporate mail, which is running on a linux server. As far as I remember, on postfix. Clients - almost all Thunderbird. Of the 50 people, 30 work in the central office, which is connected to the server by 100 Mbps. The rest are in a warehouse and in another city. They are connected to the central office with a 4 Mbps Internet connection.
The task is to remove all mail (files in which it is stored) from users' computers.
The options that I see for myself:
1. Put the mail client on the terminal server for everyone, on which they are now working with the ERP system. Cons of this method: the place on the server is not rubber and expensive (There is a SAS controller there).
2. We leave clients on cops of users, we configure IMAP. Cons: it tosses and turns for a very long time, temporary files with already downloaded letters are still stored on the computer.
3. Configure thunderbird to store its files somewhere in a shared folder on the file server. Cons: I don’t know how fast it will work, and it’s not clear what to do with remote offices that have only old linux routers from servers :)
4. Exchange. Cons: never picked it, that is, the setup will most likely take quite a lot of time. Worth the money, unlike the current implementation. Plus Outlook (without which, as I understand it, Exchange and not Exchange at all) is also not free.

Actually, on this occasion I ask for advice: which of the methods, in your opinion, is the least expensive in terms of time, finances and complexity. And which one is better.
Thank you

PS so that you understand the scale of the tragedy: managers accumulate about 4 GB of incoming and the same amount of outgoing mail per year. Marketing does too. The warehouse and other departments are much smaller.

Answer 1

5.Roundcube. Quite a chick.

Answer 2

5. GMAIL

Answer 3

I did this recently.
Thunderbird - the shared folder points to the ball by nfs. It contains all the letters.
The only thing is that I forgot why I chose nfs. Most likely due to its transparency for programs + almost native mount tools under win.

Answer 4

For good, there are only two correct options:
1. Web interface for example: RoundCube as the person above wrote
2. IMAP, there was a very good article on thunderbird automation, I advise you to go over it habrahabr.ru/blogs/sysadm/101905/

Answer 5

Are you not ready to spend money on Exchange and Outlook at all? Setting it up is not as long as you think.
And believe me: compared to this bundle, everything else proposed here is crutches.

Answer 6

IMAP+roundcube.

Answer 7

I used webmail on usermin in a similar case.

Answer 8

postfix + postfixadmin + dovecot + (rouncube/squirremail/*| thunderbird )
Thunderbird can cache IMAP mail locally, and can disable caching (Mailbox Properties/Synchronization & storage)