begun writing the section on receiving mails#

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Christoph Haas 2025-08-21 00:29:44 +02:00
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---
title: Overview
lastUpdated: 2025-08-09
slug: ispmail-trixie/overview
sidebar:
order: 120
---
import "@splidejs/splide/css";
import { Splide, SplideSlide } from "astro-splide";
Your mail server will use multiple pieces of software to do its job. I have created a little slideshow to help you
understand what happens when someone on the internet sends an email to your server. I will provide more slideshows later
when it comes to fetching and relaying emails. Use the arrows to switch between slides:
<Splide>
{Array.from({ length: 24 }, (_, i) => i + 1).map((i) => (
<SplideSlide>
<img src={`/big-picture-receive/${String(i).padStart(2, "0")}.svg`} alt={`Slide ${i}`} />
</SplideSlide>
))}
</Splide>
So:
- DNS is used to find your mail server
- Postfix receives emails using SMTP (the Simple Mail Transport Protocol)
- SQLite stores information about your domains and mail users
- rspamd checks if it is spam
- Dovecot saves it to disk

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---
title: Receive emails
lastUpdated: 2025-08-20
slug: ispmail-trixie/receive-emails
sidebar:
order: 160
---
import { Aside } from "@astrojs/starlight/components";
import "@splidejs/splide/css";
import { Splide, SplideSlide } from "astro-splide";
I have created a little slideshow to help you understand the process of receiving an email from a remote mail server.
Use the arrows to navigate between slides:
<Splide>
{Array.from({ length: 24 }, (_, i) => i + 1).map((i) => (
<SplideSlide>
<img src={`/big-picture-receive/${String(i).padStart(2, "0")}.svg`} alt={`Slide ${i}`} />
</SplideSlide>
))}
</Splide>
So:
- DNS is used to find your mail server. First the MX record. Then the A and/or AAAA record.
- Postfix receives emails using SMTP (the Simple Mail Transport Protocol)
- MariaDB stores information about your domains and mail users
- rspamd checks if it is spam
- Dovecot saves it to disk
## DNS
Let's begin with setting up the DNS records. Say that you want to receive emails for the `example.com` domain. If some
other mail server on the internet wants to send an email to your server, it looks for two records:
- **MX record (`example.com`)**
The MX (Mail Exchange) record tells other mail servers _where to deliver emails_ for the domain `example.com`.
Instead of pointing directly to an IP address, it points to a **hostname** (for example, `smtp.example.com`).
MX records also include a **priority value**. If multiple mail servers are listed, the one with the lowest priority
number is tried first. Others are used as fallback if the first is unavailable.
- **A / AAAA record (`smtp.example.com`)**
The hostname specified in the MX record (`smtp.example.com`) must resolve to an IP address so that other mail servers
know how to reach it.
- An **A record** maps the hostname to an IPv4 address.
- An **AAAA record** maps the hostname to an IPv6 address.
Once resolved, the sending mail server connects to that IP address on **TCP port 25**, which is the standard port for
email delivery (SMTP).
<Aside>
If no MX record is found, the remote mail server will instead directly look for an A or AAAA record of `example.com`.
</Aside>
<Aside type="caution">
The MX record must point to a hostname. It **must not** point to an IP address directly.
</Aside>
In zone syntax you would create something like:
```
@ IN MX 10 smtp
smtp IN A 100.64.17.3
smtp IN A fd7a:115c:a1e0::17
```
If possible, also add a PTR record that makes the IP addresses point back to the name `smtp.example.com`. Other mail
servers may be more likely so flag your sent emails as spam if you do not have a PTR record. Preferable the forward
(A/AAAA) and reverse (PTR) records match.
## Postfix