Looking for a guide to collect all e-mail from the ISP mail server

[Peter]

Am 26.10.20 um 11:24 schrieb R. Diez:
> 
>> Hello R, I only wrote about the incoming side - of course, you also 
>> want to
>> send mail to remote users, and that includes users with an address of 
>> …@myisp.com. They will go to the ISP and be fetched to local from there.
> 
> 
> That is not what I had in mind. My users will not go to the ISP and 
> fetch their e-mails from there. They will always go to my internal mail 
> server. If a user is on the road, he/she will connect with OpenVPN first.

Probably I could have said that better: fetchmail will fetch those mails 
from the ISP, same as any other mails to someone at your.site - the Inbox 
at your ISPs will always be empty, your users will only interact with 
the dovecot instance on premise. There is some inefficiency, the price 
for a simpler setup.

> I have seen Microsoft Exchange setups that carried on working locally if 
> the Internet connection was down. If Microsoft can do that, I want to 
> have it too. 8-)

>> With some tinkering, you can configure your local relay smtp to 
>> deliver those locally,

To be more clear - if you have a local smtpd too (not just dovecot and 
fetchmail, postfix perhaps), that sits between your users MUA and your 
ISPs smtpd, you can make it recognise someone at your.site as a "local" 
account and have those mails delivered locally. You have to set up some 
mappings though, that replicate the ones in your fetchmailrc.

Start of a HOWTO:

1) Install dovecot, create virtual accounts for all of your users
2) Install fetchmail, make it pull the ISPs IMAP and deliver locally
3) Install postfix as a smart relay and deliver locally to locals

Feel free to fill in the details ;)

-- 
peter