Hello R,
Your goal does not sound weird. The most painless way might be to fetch incoming messages from the ISP's IMAP and deliver them to your local dovecot. A shortened fetchmailrc would read:
poll remote.server … user …, password … folder 'INBOX' fetchall idle ssl mda "HOME=%T /usr/bin/sudo -u %T /usr/lib/dovecot/deliver"
That way your users can create their vacancies with the ISP portal, the ISP will do availability, antivirus etc. You can even use sieve on delivery. Perhaps fetch "Spam" too, if your ISP files it away.
Beware, you have to somehow keep tabs on remote and local usernames. Passwords will be different. Local updates should be no problem with a reasonable distro, e.g. the dovecot public repo.
Happy becoming a mail server admin!
Peter
Am 25.10.20 um 18:56 schrieb R. Diez:
Hi all:
I am evaluating mail server solutions for a small business. The trouble is, I am only a part-time admin and a newbie to mail servers.
Most guides I have seen are rather unrealistic: they encourage you to expose your e-mail server to the Internet, and hope that you have the resources to keep it patched up.
I would rather have an internal mail server that collects e-mails from a standard ISP mail server. It is like the old "POP3 Connector" that came with Microsoft Exchange. Sometimes, there is a mailbox per user on the ISP, and a corresponding one on the local server. Other times, there is a single "catch all" or "multidrop" mailbox on the ISP.
Users can still access their internal mailboxes from outside through an OpenVPN connection. The goal is that only VPN, and perhaps SSH, are accessible from the outside. We do not need to arrange any special SMTP configuration with the ISP either.
This kind of mail server setup is rather different to the standard configuration. You do not normally need you own antivirus and spam filter, and you do not need to configure SSL certificates, MX or SPF DNS records. Most ISP handle that correctly and economically. Internal e-mail does not leave your LAN, and your internal SMTP server is just a relay for the external ISP SMTP server.
Furthermore, most guides do not explain how to setup an autoresponder ("I am on holiday until xxx") so that users can enable theirs with the mouse. Editing configuration files over SSH is not really an option for normal users. This detail is important because it could be the only thing I need above standard e-mail. Further groupware features can be seen as nice but ultimately unnecessary luxury, and a basic shared calendar can be accomplished with a separate server like https://radicale.org/ and a calendar client like one built into Thunderbird. Hopefully, that is all I would need for a small business.
Can anyone point me to the kind of guide I need? Failing that, I would need information or examples about using fetchmail, getmail or similar software with Dovecot. Good or bad experiences from you guys would also help.
Each of those tools has a detailed man page, but there are many options and ways with different advantages and disadvantages. I would need a simpler guide to get started.
I am aware that there are pre-packaged mail server solutions that would perhaps bring an easy-to-use autoresponder, but I haven't seen one yet that where you could tick a box like "this server is only internal and collects mail from the ISP server" during installation. Nor have I seen instructions about reconfiguring the mail server for my ISP mail scenario.
I am prepared to learn more and write my own Perl scripts and/or installation guide, but it would be stupid to waste time if something easy already exists. After all, the setup I am describing (external ISP mail server + internal mail server) is not so weird.
Thanks in advance, rdiez