dovecot - Dec 2018 - Downloading e-mail from a master mail server

Hi all:

I am trying to migrate a small company from Microsoft Exchange / Outlook 
to Thunderbird. I am evaluating e-mail server software.

We have an Internet provider that we do not really want to rely upon. We 
also do not have the resources to maintain a mail server visible from 
the outside, especially regarding security updates.

Each employee has an e-mail account on our current Internet provider, 
and one e-mail account on our internal Exchange Server. Our Exchange 
server periodically downloads all e-mails from the Internet provider 
mailboxes to its local mailboxes. Therefore, employees cannot access 
their e-mails when travelling.

We have worked this way for years without problems. There are other 
ways, like VPN access etc, but we do not have the resources to install 
or maintain more IT stuff.

So I would like to keep this setup, with one important difference: 
e-mails on the Internet provider mailboxes should remain there for a 
couple of weeks. This is similar to Thunderbird's option "Leave a copy 
on server" together with "For at most xx days". This way,
employees
could access at least their most recent e-mails when travelling, if only 
over the Internet provider's web interface.

I could achieve such a setup without a local mail server at all, only 
with the Thunderbird clients, but I have not figured out yet how to 
automatically backup all mailboxes. See this question of mine:

https://support.mozilla.org/en-US/questions/1243605

So I am trying to design a solution with Dovecot, but I know too little 
about mail servers. How could I configure Dovecot / MTAs / whatever in 
order to achieve the "Leave a copy on server" together with "For
at most
xx days" mentioned above? This way, Dovecot does not need to be exposed 
on the Internet.

Failing that, could someone tell me at least how to configure Dovecot / 
whatever to download the external IMAP mailboxes to the internal IMAP 
mailboxes? When I read about Postfix and the like, I see SMTP and e-mail 
queues, but that's not what I need.

This guy wants a similar setup:

https://serverfault.com/questions/500591/fetch-all-mail-from-an-imap-server-with-postfix-to-distribute-on-local-imap-serv

But I did not understand the Fetchmail part, which seems to be 
discouraged anyway:

"Fetchmail can be used as a POP/IMAP-to-SMTP gateway for an entire DNS 
domain, collecting mail from a single drop box on an ISP and 
SMTP-forwarding it based on header addresses. (We don't really recommend 
this, though, as it may lose important envelope-header information. ETRN 
or a UUCP connection is better.)"

I also haven't understood yet the backup part with Dovecot. There is no 
central e-mail database like in Microsoft Exchange, right? How do I 
backup all mailboxes for all users? I probably need to stop the Dovecot 
server an any MTAs before backing the raw files underneath, right?

Many thanks in advance,
   rdiez

>I am trying to migrate a small company from Microsoft Exchange / 
Outlook 
 >to Thunderbird. I am evaluating e-mail server software.

Why don't stay at exchange? I have been testing a redundant setup with 
2016 for a while and it is do'able. If you are not using public folders, 
you have lots of clients that interact quite well with it. Eg. macos 
mail syncs tasks and notes with the native applications remindes and 
notes.

 >We have an Internet provider that we do not really want to rely upon. 
We 
 >also do not have the resources to maintain a mail server visible from 
 >the outside, especially regarding security updates.

Get one you do rely upon. I can't imagine you do doing a better job in 
most cases if you do not know dovecot, do not read the manuals here, do 
not have a test environment. Because you do not have the 'resources'.

 >Each employee has an e-mail account on our current Internet provider, 
 >and one e-mail account on our internal Exchange Server. Our Exchange 
 >server periodically downloads all e-mails from the Internet provider 
 >mailboxes to its local mailboxes. Therefore, employees cannot access 
 >their e-mails when travelling.

Yes the pop connector in exchange 2003. We have one running still in a 
firewalled environment and limited l2tp vpn access. Maybe you can switch 
this to some spla(?) licensing program of ms, so you pay per account?

 >We have worked this way for years without problems. There are other 
 >ways, like VPN access etc, but we do not have the resources to install 

 >or maintain more IT stuff.

Your provider should be able to facilitate you with a vpn gateway, to an 
'offline' exchange server, that is not really a big deal. 

 >So I would like to keep this setup, with one important difference: 
 >e-mails on the Internet provider mailboxes should remain there for a 
 >couple of weeks. This is similar to Thunderbird's option "Leave a
copy

 >on server" together with "For at most xx days". This way,
employees
 >could access at least their most recent e-mails when travelling, if 
only 
 >over the Internet provider's web interface.

So configure your pop connector not to delete after download. You will 
still have the problem with users creating online imap folders that the 
popconnector cannot see. Maybe you can solve this with an imap sync 
program. But this is a wrong direction to take.  

 >I could achieve such a setup without a local mail server at all, only 
 >with the Thunderbird clients, but I have not figured out yet how to 
 >automatically backup all mailboxes. See this question of mine:
 >

Configure clients to store mail files on the a network share, and do 
backup on the server? But also this is thinking in a wrong direction. 


 >
 >So I am trying to design a solution with Dovecot, but I know too 
little 
 >about mail servers. 

So study it, read about it. How can you select a good service provider, 
without this knowlegde?

 >How could I configure Dovecot / MTAs / whatever in 
 >order to achieve the "Leave a copy on server" together with
"For at
most 
 >xx days" mentioned above? This way, Dovecot does not need to be 
exposed 
 >on the Internet.

If you have to ask this question, you should not be doing this. 
Especially if you value your security so much, as you mentioned before.


 >Failing that, could someone tell me at least how to configure Dovecot 
/ 
 >whatever to download the external IMAP mailboxes to the internal IMAP 
 >mailboxes? When I read about Postfix and the like, I see SMTP and 
e-mail 
 >queues, but that's not what I need.

Here is described how you configure dovecot.
https://wiki.dovecot.org/


 >I also haven't understood yet the backup part with Dovecot. There is 
no 
 >central e-mail database like in Microsoft Exchange, right? How do I 
 >backup all mailboxes for all users? I probably need to stop the 
Dovecot 
 >server an any MTAs before backing the raw files underneath, right?
 >

If you do not have time to read, get some help. Otherwise you will 
create a mess for your users. The problem with IT is that everyone is 
just jumping in without education nor responsibility. 
It is like your dentist is earning something on the side in the weekend 
as a brain surgeon.
Start acting like a pro