Linux – DIY Linux email journalling system

You're best off sticking to "standards" as much as possible, in my eyes that means;

• MTA: Postfix
• IMAP: dovecot
• Auth/routing/etc: OpenLDAP
• Mailing lists: mailman
• Spam/virus: amavis + clamav + $commercial_scanner + SpamAssassin
• Webmail: Squirrelmail is probably your best option. You could try Roundcube.
• Calendaring: just use Zimbra if you want calendaring.
• Mailbox storage: Some flavour of maildir, on the filesystem. Don't use a database. Metadata is fine to live in a database but messages belong on the filesystem.
• Proxying/routing inbound requests: nginx or perdition
• HA: that's more complex. DRBD + heartbeat between a pair of servers for the backend mailbox storage.

Don't use unmaintained/out of date software, and don't install this kind of stuff from source - use your distro's packages. Your future sysadmins will thank you.

Using obscure software just makes it that much harder for the next person to maintain the system. Brian is spot on with his comment above.

Interested to know why Zimbra was discounted? It is open source after all. I've migrated a couple of "roll your own" installs onto Zimbra and it's much nicer than doing the integration yourself. Zimbra is worth it for the web interface alone.

I've been running my own email server for 17 years, including hosting a large mailing list, and although it takes time I find it's perfectly possible, even today.

Most of what should be done, you're already doing; but one factor you don't address is what kind of IP address you're using. Many mail providers outright blacklist (or use as a strong marker for spam) IP address blocks which traditionally act as spam sources. First and foremost, those have been the domestic broadband address blocks, which if directly sending email are almost always part of zombie botnets. But in this case I note that your server has a Hetzner address, and is therefore very likely a VPS:

[me@anni ~]% whois 144.76.81.247
[...]
inetnum:        144.76.0.0 - 144.76.255.255
netname:        HETZNER-RZ-BLK-ERX1

VPSes are disposable, and a perfect target for senders who want to fire up a server to send out a couple of hundred million spams then disappear again. I imagine that at least some big providers have taken to adding VPS blocks to those internal marker/black lists.

My mail server is a physical box, painfully and expensively hosted at a colo facility, and possessed of some IP addresses from a range reserved for real server colo (or, more precisely: a range not known to be used for domestic or VPS purposes). These are much less likely to be regarded as structurally tainted, because it's much more trouble and expense to set up a physical server than a VPS, so a spammer is much less likely to do it.

So I guess my precise answer would be that it's still perfectly possible to run your own SMTP server, but it may require more pain than you have yet undergone. Sorry.