Emil Lerch

Husband, Father, Technologist, Cloud Architect

SMTP and email notes

SMTP and email notes

This is a general post to document what I’ve been learning while setting up my own email server. It’s been a long time since working with SMTP directly, especially with things like SPF, DMARC, and DKIM. A few general feelings of this process:

  • It’s not terrible! Running your own server is something that general Internet advice says DO NOT DO. But that advice, at least so far, seems wrong. I can send to gmail without issue, and get 10/10 on mail-tester.com
  • There’s a lot of bad advice on the Internet
  • The system is not magic
  • It’s pretty flexible
  • Security has improved a lot since the days of “telnet to port 25”!

Let’s start with security, because with all the spam out there, email kind of begins and ends there


Let’s start with “telnet to port 25”. Port 25 is in use, but connections can be TLS encrypted over that port. From the server side, we can require it. Also, authentication of course. TLS on all the other ports, and encryption in transit is covered.

Connecting to other servers, services also check other things. In order of importance, I find quite a bit of poor advice on the Internet.

  • Reverse DNS, or PTR records

Unlike what I read before hand, these are critical. Basically, you can have a near perfect score, and nobody will deliver your email without this. Of course, it’s the owner of the IP range that needs to do this, so setting that up will vary based on the hosting provider you use.


Only DKIM is marginally a problem. SPF, and especially DKIM, provide indications of the integrity of the mail. DMARC is simply there to define your policy of what should be done should SPF or DKIM fail. That’s the high level, but a good first read on these technologies is here on Cloudflare.

DKIM is the most finicky, mostly because you have to have everything aligned for it to work. But, it’s basic cryptography. You store the public key in DNS, along with a “selector”, and your server software signs the email with the private key. The email header describes what other headers are part of the signature, so your email can be sent through 10 relays and it doesn’t matter at all unless one of them messes with those headers or the email itself.

The most fun thing IMHO is that selector. I was initially confused why that’s needed, but the answer is relatively simple. If you use multiple servers or services, you don’t need to pass multiple keys around. You can also have multiple private keys to do rotation. The server provides the selector in the DKIM header, so the domain lookup will find the right DNS record for that particular email. More on this later, but it makes things pretty flexible.

  • Hosting provider

This gets a bit tough. The big cloud providers all have various restrictions on using their servers to send email. I’m sure they want good IP reputation (see below), but I suspect other reasons exist and I don’t want to speculate too much. I believe DigitalOcean and Vultr will allow it, but ultimately I found George DC had a pretty good deal, and is fine with sending email, as long as it’s not for spam. I even submitted a ticket to update the reverse DNS and it was done in about an hour.

  • IP reputation

I haven’t had to worry about this too much, but I did notice two or three hits on IP reputation lists. They were old and it probably doesn’t matter, but I wanted to know how to clean that up. It’s both easy and painful. Easy, because you typically just need to go to the place that claims you have a nefarious IP address and click a button to delist. Painful because you have to do it at every place that reports your IP address, and many of those sites need you to establish an account. One required me to submit a ticket (which seems to have been handled without human intervention). Note that I did not try to use my home Internet connection for any experimentation - the Internet says that’s a really bad idea, and I tend to agree it’s probably not worth trying.


I’m using docker-mailserver, which sets up most things for you. At the moment they seem to be transitioning some of the component software, but the docs were reasonable and it’s possible to make basically whatever configuration changes you need. They don’t hide their component software, so if you want to make changes, you’ll be digging into postfix, dovecot, etc. documentation. But, postfix in particular is a good battle tested server that is extremely flexible. Your mileage may vary, especially since this does not include any kind of web-based email interface. I don’t really need that, but if you do, there are other alternatives to consider like mailcow. I cannot vouch for anything else though - if I want web-based, I can just connect my nextcloud server’s mail app to it through IMAP.

Advanced terminology and use cases

I’m interested in something called “split routing”, where one mail server handles some email, and forwards the rest to another mail server. SMTP is actually very decentralized, though, so the idea of “this is your mail server” isn’t actually really a thing. Yes, there is a mail server that hosts your mail box, but really, anyone can do anything. The flexibility can be confusing, but it’s also pretty powerful.

One thing that I have really comes to terms with is the idea of an MTA, or mail transfer agent. Typically, we talk about mail servers, but technically, they are mail transfer agents. A mail comes in, and if the software knows that the mail is stored locally, that’s where it transfers it to. Otherwise, it determines the next place to go to. Whether that mail comes in from an IMAP connection, SMTP, or another SMTP server, is immaterial really. It just matters “where does this mail need to go”. In postfix, this means that all we need to do for “split routing” is define overrides in transport maps for emails we want to override. Literally…that’s it. The stuff we don’t override goes to where it’s supposed to go to. With a lot of help from LinuxBabe, I set up split routing, and it basically looks like this:

  • My DNS MX record is my server
  • I signed up with kingmailer.org for a trial. They tell me to go set up an MX record. I IGNORE that.
  • I set up a kingmailer@ email address on my domain, in kingmailer.
  • I set up a user@ email address on my domain, but only on my own server

Note, that at this point, kingmailer doesn’t know there’s another mail server, and does not know about user@ email address. Right now, the server that DNS points at, which will handle all email, has no idea of a kingmailer@ email address. Time to make it aware.

docker-mailserver has a mechanism for overrides, so this bit is specific to that software, but I simply dump a postfix-main.cf into the configuration directory, with a single line:

transport_maps = texthash:/tmp/docker-mailserver/postfix-transport_map.cf

docker-mailserver default docker compose file will mount a config volume in that location. The transport map itself is also relatively simple:

[email protected]     smtp:[kingmailer.org]

Inbound emails will still be rejected though because there is no local account, so using docker exec -it mailserver setup add email [email protected] makes quick work of that. All that script does though is add a line to the account configuration and ask postfix to reload.

…and…that’s it! Because I specify the server here (with square brackets to prevent more processing), my server will not bother looking at MX records, it will just send the mail to the right place. LinuxBabe recommends adding your domains into this map with a value of local, and I played around with that, but kept getting errors. Leaving it out seems to work just fine, and my assumption is that the default, at least in docker-mailserver, is not a local transport.

One thing I really wanted to do, but again, it goes against the general wisdom of the Internet, is get a unified inbox in my actual email client. I’ve gone back to mutt (actually, neomutt) which has the disadvantage of that HTML email is inconvenient, but also has a strong advantage in that HTML email is inconvenient. So many trackers, etc that are no longer automatically triggered. I’ve been working with neomutt over IMAP with gmail for over a year now, and rarely need the actual web version. “Clicking links” is something that’s not as pleasant as I would like, but all in all, I’m not sure HTML email was all that great an idea in the first place. ;-)

Mutt/Neomutt don’t really handle a unified inbox very well, so I figured out a way to do this by syncronizing my postfix data directly into my mail directory, thus violating not one, but two pieces of generally accepted wisdom. First, “you really don’t want to have two accounts in the same mail directory”, and second, “don’t use syncthing to synchronize a subdirectory of a synchronized directory”. The latter came as a warning message directly in the UI of the software, so that was a little scary, but I’m ok… more on this in a second.

The first advice was legitimate. My mail directories use isync (aka mbsync) to synchronize all my imap folders in Gmail to my local storage. If I’m going to keep the mail separated, I don’t want my postfix mail to get uploaded, so I needed mbsync to ignore the postfix mail. I also needed syncthing to ignore gmail-sourced mail. I have decades of email in gmail, and I do not need that on my VPS with only 40GB disk. For the first problem, I actually forked the isync source code at the latest release, then added a 3 line patch to look for a substring of the file name, and ignore it. As all postfix-sourced email ends up with the server host name in the file name (but Gmail does not), I could use that to let mbsync ignore my postfix data. On the syncthing side, I used the following ignore pattern:


That first pattern looks nuts, but I basically tried to scope to exactly what matched the mbsync stuff. The dovecot line is due to some dovecot accounting files (I’ve been saying postfix, but the files are actually managed by dovecot, which also handles imap).

The last issue is the syncthing warning about syncing a subfolder of a synced folder. This is ok in this circumstance. I’m sharing my overall mail folder with every device I want to use neomutt on (this allows me offline access when I’m traveling). However, just my inbox folder/subfolders is being synced to my mail server, and shouldn’t be shared anywhere else. Mutt/neomutt can happily think it’s working with a local mail directory, while everything I do is transparently synced back to gmail (for gmail-based email), or to my mail server (for email stored there).


I mentioned kingmailer a couple times. I’m sure there are a bunch of reasonable services, but kingmailer had a 14 day trial that did everything needed to get comfortable with the process.

Debugging DKIM (is my DNS updated properly? Am I loading the right private key? What about that bug in docker-mailserver where the config doesn’t persist properly? Is docker-mailserver using OpenDKM or rspamd?) was a bit painful. I found https://appmaildev.com/en/dkim pretty useful. It’s a bare bones site, but is not restricted to three tests a day the way mail-tester is. They also have SPF and DomainKeys tests, but I didn’t need them. For a more general test (everything, not just DKIM), I used https://www.gmass.co/analyze, which also avoids the mail-tester restrictions. Eventually though, you really need to test with https://www.mail-tester.com, because that seems to be the gold standard for determining proper setup. If you’re scoring 10/10 on that site, which you really should be, and your email isn’t going through, the problem is them, not you.

All inbound email is managed via MX records for the domain. I use Cloudflare for that.

Georgedatacenter, as mentioned above, has been good so far. Debian and docker provide the remaining software stack.


After testing this process thoroughly, I’ve moved the lerch.org mail to use this set up. Now I have docker-mailserver handling my own email (inbound and outbound), and push the rest of my family off to gmail. If the server goes down for some reason, the fall back MX (mail server, mail exchange), remains gmail, so they will pick it up. Any email I try to send will fail until I reconfigure or bring my server up, but that’s ok by me.

It is interesting to see the connections from various servers coming through my new mail server. There sure is a fair amount of spam. But so far (only a couple hours, granted), I’ve yet to see any spam get to my inbox. I’ll post an update if this all turns out to be a terrible idea, but so far I’m looking at all this advice on the Internet “you don’t want to host your own mail server” and saying, “really? This doesn’t seem too bad.”.

If you’ve followed along this far, I assume you’re curious, so here’s a full diagram of my email setup. I still have some fine-tuning to do, but I’m pretty happy with it. One note - another piece of advice you see a lot is to use notmuch to allow easier searching. I find this less than optimal for two reasons. 1) your inbox in (neo)mutt becomes effectively read only, and 2) the search index is a massive single file, so synchronizing it is a bit painful. I instead 95% rely on (neo)mutt’s built-in search, which is nice and fast if you stick to what’s in subject or headers (just from?), and use a special command that performs a notmuch-based search on my server if I need something fancy. The server index is kept up to date as email comes in.

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