Hosting my own SMTP server

I discussed previously how I found myself searching for a way to provide email notifications from my servers. I had previously been using my Gmail account, and I realized I didn't want the servers to have access to my Gmail account any longer.

Since I already had a domain name, I wanted to try out hosting an SMTP server. While the idea of self-hosting the SMTP server seemed crazy to me, for this limited use-case it actually turned out well and was relatively easy! Being counted as spam hasn't been too large a problem and the configuration wasn't too bad.

The Apprehension

I had previously hosted my own email server in 2004. And I hated it. Outgoing emails could easily be marked as spam and incoming emails were virtually all spam. It wasn't serving much purpose, so I had turned it off.

Mail transfer agents (MTAs), which is what I'm needing here, are also notoriously obscure and complicated to configure properly. There are also many to choose from and there's generally no "cookie cutter" configuration, so the task requires learning a reasonable amount each option in order to determine a preference. I'm maybe willing to learn one, but I'd rather not sink a bunch of time into an option just to discover I hate it. Or that I hate all the options.

I had also expected things had gotten worse since I last delved into it, since there's been new standards to address spam and I have little idea how they work, although I know some use cryptography. Given how painful TLS is to configure (and how painful it was to get a certificate before Let's Encrypt), that sounds awful.

Virtually everyone on the Internet agrees that the best way to avoid ending up in the spam folder is to use a well-established email provider. I need this for possibly-important notification emails; I will have a very low tolerance for them going to spam. I worried even after all the effort of setting things up, too many emails go to spam and I have to scrap it.

SPF (IP Whitelist)

As part of my research, I had taken took a quick glance at SPF. It was easy enough that I set it up immediately. SPF is simply a whitelist of IP addresses that send email for your domain, and published via DNS.

The detailed record syntax is a good reference after you see an example. You publish it as a TXT record for your domain. So I had a TXT for @ for my domain. A simple value to say "mail should come from IP" would be v=spf1 ip4: ~all.

Since explicitly listing IP addresses is annoying to update, you can refer to a hostname with something like You can also refer to servers in your MX records with mx. And you simply list multiple things before the ~all to allow them, like v=spf1 ip4: ip4: mx ~all.

If you want to see published values for various domains (or your own), you can use dig via dig txt.

You should probably avoid the stricter -all. Legitimate email forwarding by others breaks SPF. For example, I can use Google Domain's email forwarding feature to forward to my Gmail account. If you send an email to, that email would be forwarded to my Gmail account by a Google Domain server with you as the sender. But your SPF policy doesn't allow Google Domains to send emails for your account, so your SPF will fail because of my configuration. And if you have DMARC enabled, you'd receive failure reports. This is a fatal flaw of SPF. And so we limit the negative impact of SPF with ~all.


As I glanced at various SMTP servers, I came across a two-line configuration snippet on the Arch Linux wiki for an OpenSMTPD server acting as a mail relay. It seemed pretty comprehensible so trying out OpenSMTPD seemed worth a try. And it turned out great.

The smtpd.conf documentation is really nice. Between the default smtpd.conf and it I quickly had my initial configuration, which was simply:

listen on localhost

table white-sender {""}
table white-recipient {"", ""}

accept sender <white-sender> for any recipient <white-recipient> tls verify

I was very concerned about being marked as a spammer, so I wanted the white lists before any testing. It would have been very easy to have test emails that were "bad" (like wrong "from" address) and I didn't want those to go to Google. So the configuration only listens on localhost and only allows outgoing mail from addresses to the whitelisted Gmail account or to my domain.

I needed a test command. I used:

echo "Subject: sendmail test; unique-test-desc" | \
    sendmail -v -f

And it failed! But with a helpful link to Google's Guidelines for IPv6. I was actually really happy to see this, because the documentation was really helpful for more than IPv6.

So the problem was that IPv6 requires PTR records. While I may mess with that eventually, I don't want to do it out-of-the-gate. So I limit myself to using IPv4 by adding a configuration line: limit mta inet4. And then my test email arrived! (Note: I already had SPF configured here; I don't know how important it was.)

At this point there's an email in my inbox that I can inspect. In the email drop-down menu I clicked "Show original" to see the raw MIME email. This was helpful to see that the sender and receiver were correct. But looking at "Authentication-Results" was also helpful. In this email, I saw a "spf=pass" with some additional information that is useful for debugging. I was also able to confirm TLS was used by seeing "version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256" in the "Received"). When you get there, you can also use it for DKIM and DMARC debugging. If you are new to email, understand that each server the email passed through will prepend additional headers. So, for example, there are generally multiple "Received" headers, and the topmost one was added by the last server.

That was really easy! Now it was iteration and improvement.

DKIM (Cryptographic Message Signing)

DKIM is a signature-based mechanism to prove that an email originated from a particular source. Interestingly, DKIM can be used to sign emails even if the email being signed is from a different domain. This allows Gmail, for instance, to send an email impersonating an address, but the DKIM would show it coming from a server. So the receiver would then have a clear view: did not send the email, but if is reasonably trusted, it can be accepted anyway.

DKIM publishes public keys via DNS as a subdomain of the _domainkey subdomain. The creation and publishing of keys is fairly easy, especially compared to things like TLS. Most of the effort comes from configuring your server to provide the signatures using they key, and that's not onerous.

OpenSMTPD provides an example using dkimproxy and dkimproxy provides reasonable documentation. Note that Google suggests using 2048 bit keys, so you may want to use "2048" instead of the "1024" in the "openssl genrsa" command when following the dkimproxy documentation.

I used a dkimproxy_out.conf like:

keyfile   /etc/dkimproxy/private.key
selector  example

"Selector" would match the DNS name used, so "" (which doesn't exist) in this case. You can use dig via dig txt.

Unlike the example documentation, I'm using filtering, so I found it useful to use tagged ! DKIM since otherwise it seemed possible to form an infinite loop:

listen on localhost
listen on localhost port 10028 tag DKIM
accept tagged DKIM sender <white-sender> for any recipient <white-recipient> tls verify
accept tagged ! DKIM from local for any relay via smtp://

Note that I'm filtering after adding the DKIM signature. Since it is possible for someone on the same machine to write to dkimproxy directly, I am considering the results from the proxy "untrusted."

DMARC (Enforcement and Reporting)

DMARC specifies what to do with emails that have SPF or DKIM issues and how to report them, published via DNS. For instance, if you misconfigured your SPF and Google was flagging emails, you'd like to be notified instead of just happening to notice emails were being rejected. While the DMARC standard allows for multiple types of notifications, the main option is to specify an email address that should receive aggegrated daily digests independent of whether there are problems. Since that'd be annoying to go to a personal email account and you really need to process the raw data, there are services that you can route these emails to and can provide pretty graphs.

DMARC also uses a TXT record in DNS, but with the _dmarc subdomain (so for me). If you are comfortable with your setup, a strict value would be v=DMARC1; p=reject; with a proper email address. The standard disallows reporting emails to go to a different domain, but that is different from an earlier draft and I suspect servers may allow it anyway. But I've also not needed to investigate this for my set up. I found G Suite's documentation to be most clear in describing the useful options.

If you want to see published values for various domains (or your own), you can use dig via dig txt.

Final notes

To support extra machines, you can either set up each with it's own server (and its own DKIM selector), or you could allow them to use the first machine's SMTP server. I went with the latter option. I considered using client certificates, but simple username/password seemed about as effective and easier to setup and manage.

If using Let's Encrypt for TLS certs, note that OpenSMTPD is particular about the permissions of the private key file. You can fix the permissions via chmod o-r /etc/letsencrypt/live/ You can configure /etc/letsencrypt/renewal/ to include a renew_hook within [renewalparams] to automate the "fix".

Also, if you're interested in using file-based tables in OpenSMTPD, I'll note that smtpctl update table <tablename> didn't actually work for me. I just restart OpenSMTPD instead.