---
title: TLS certificate
lastUpdated: 2025-11-01
slug: ispmail-trixie/tls-certificate
sidebar:
  order: 158
---

import StepListReceive from "../../../components/StepListReceive.astro";
import { Aside } from "@astrojs/starlight/components";

<StepListReceive currentStep={2} />

Your mail server can't work properly without a valid TLS certificate. It will be used in three places:

1. the **webmail** interface (driven by the Apache web server)
2. **Postfix** (to encrypt SMTP communication with other mail servers on the internet where possible)
3. **Dovecot** (to encrypt IMAP/POP3 communication)

Your mail server will be able to host email addresses for many domains. But a TLS certificate is usually issued for a
single [FQDN](https://en.wikipedia.org/wiki/Fully_qualified_domain_name) like `mail.example.org`. That _common name_ is
an important part of the certificate. It must match the name your users use to talk to the server.

**Correct**: Your mail server is addressed as `mail.example.org`. The TLS certificate was also issued to
`mail.example.org`.

**Wrong**: Your mail server is addressed as `smtp01.example.com`. But the TLS certificate was issued to
`mail.example.org`.

So choose an FQDN for your mail server. For example I use `webmail.workaround.org` for my personal server.

## Certbot

You already have the `certbot` software and the `apache2` web server installed. Requesting a certificate from Let's
Encrypt is as simple as running the next command.

<Aside type="danger" title="Important">

Please replace `mail.example.org` by the FQDN you chose.

</Aside>

```sh title="Run this on your server"
certbot --apache --register-unsafely-without-email --agree-tos -d mail.example.org
```

<details class="collapsible">
  <summary>Click here to learn what the options mean…</summary>

- **--apache**: Use the [HTTP challenge](https://letsencrypt.org/docs/challenge-types/) to verify that you are actually
  in control of the FQDN. The certbot plugin will add some configuration to intercept HTTP requests to
  `/.well-known/acme-challenge/…` on your web server. This is a fully automatic process that does not even have to stop
  your web server.
- **--register-unsafely-without-email**: Do not ask for an email address to send to Let's Encrypt. Before June 2025
  Let's Encrypt used that address to inform you if your certificate is about to expire and the automatic renewal failed.
  Nowadays they do not need an email any more. But for historical reasons they still ask for it by default.
- **--agree-tos**: Confirm that you have read Let's Encrypt's terms and conditions. Let's be honest – nobody reads that.
- **-d**: The domain that the certificate will be issued to. This has to be the exact FQDN of your server.

</details>

The output should look like:

```
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Requesting a certificate for mail.example.org

Successfully received certificate.
Certificate is saved at: /etc/letsencrypt/live/mail.example.org/fullchain.pem
Key is saved at:         /etc/letsencrypt/live/mail.example.org/privkey.pem
This certificate expires on 2025-11-22.
These files will be updated when the certificate renews.
Certbot has set up a scheduled task to automatically renew this certificate in the background.

Deploying certificate
Successfully deployed certificate for mail.example.org to /etc/apache2/sites-available/000-default-le-ssl.conf
Congratulations! You have successfully enabled HTTPS on https://mail.example.org

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
If you like Certbot, please consider supporting our work by:
 * Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
 * Donating to EFF:                    https://eff.org/donate-le
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
```

<details class="collapsible">
  <summary>Click here to learn which files have been created…</summary>

In /etc/letsencrypt/live/**mail.example.org** you will find a couple of files now:

- **cert.pem**: the certificate file
- **chain.pem**: the _chaining_ or _intermediate_ certificate. This certificate provides information how the LetsEncrypt
  certificates are linked to other known certificate authorities. It is generally a good idea to always send this
  certificate along with your own for clients who may not know LetsEncrypt properly yet.
- **fullchain.pem**: this file contains a concatenation of the _cert.pem_ and the _chain.pem_. This is the preferred
  file to use when a piece of software asks where to find the _certificate_.
- **privkey.pem**: the private key file. Keep it secret.

Well, actually those are not files but _symbolic links_ pointing to the "archive" directory. But don't worry about that.

</details>

## Renewal

Your new certificate is only valid for 3 months. Fortunately the renewal is happening automatically. A _systemd timer_
checks your certificate frequently and triggers a renewal one month before it expires.

It is important that services that use the certificate load the new certificate after an automatic renewal. This shell
command will create a file `/etc/letsencrypt/cli.ini` that handles it:

```sh title="Run this on your server"
cat > /etc/letsencrypt/cli.ini << 'EOF'
# Restart services after renewing a certificate
post-hook = systemctl reload postfix dovecot apache2

# Because we are using logrotate for greater flexibility, disable the
# internal certbot logrotation.
max-log-backups = 0

# Adjust interactive output regarding automated renewal
preconfigured-renewal = True
EOF
```

Perfect. You won't have to worry about the certificate again.

<details class="collapsible">
  <summary>Click here to learn how the renewal works…</summary>

These commands will show you the definition and status of the timer and the renewal service:

```sh title="Run this on your server"
systemctl status certbot.timer
systemctl cat certbot.timer
systemctl status certbot.service
systemctl cat certbot.service
```

_Systemd timers_ are similar to cron jobs. They require more work to set up but have some nice features and make you
look cooler.

</details>
