EDIT: Tests fine now, thanks!

See https://docs.cloudron.io/email/#autodiscover

curl -v https://domain.com/.well-known/autoconfig/mail/config-v1.1.xml

That seems to have done the trick. Thanks!

Is there supposed to be a setting that I need to enable in domains > advanced ? Because I don't see anything related to this in the settings.

https://git.cloudron.io/cloudron/box/-/commit/ef68cb70c07f2abd524879aef70930b572f48917

This looks like it should simply be a checkbox in the UI, and Cloudron manages the ~yellowtent/boxdata/well-known/<domains> hierarchy.

Yes, correct. Except I broke this because I completely forgot about autoconfig well known support and only implemented mastodon and matrix Will get it fixed for 6.1.2.

Do I create a similar config-v1.1.xml files on autoconfig.domain2.com, autoconfig.domain3.com etc or can I somehow edit the autoconfig.domain1.com/mail/config-v1.1.xml to include the additional domains?

My understanding is that you have to create separate configs i.e your former suggestion. After all, there is no way of knowing domain2's autoconfig is in domain1.

Was that the plan @staff ?

I am also hosting other domains (domain2.com, domain3.com), on my cloudron server and was wondering how I could set autoconfig for them as well.

Do I create a similar config-v1.1.xml files on autoconfig.domain2.com, autoconfig.domain3.com etc or can I somehow edit the autoconfig.domain1.com/mail/config-v1.1.xml to include the additional domains?

Let's say you have the following domains:
test.com: The domain for which you want to have email autoconfig
my.test.com: The URL of your cloudron server that manages your email and will broadcast the autoconfig configuration.

setup DNS entries in the test.com DNS

// entry for outlook (never tested since I only use thunderbird)
_autodiscover._tcp 600 IN SRV 0 0 443 my.test.com
// entry for thunderbird and others
autoconfig 600 IN CNAME my.test.com
smtp 600 IN CNAME my.test.com
imap 600 IN CNAME my.test.com

// autoconfig.test.com does not need an SSL certificate
// smtp.test.com, imap.test.com must have an active SSL certificate

Create the mail directory and autoconfig file

in the webroot for autoconfig.test.com create the file

mail/config-v1.1.xml

Content of mail/config-v1.1.xml

// Here is a sample:

<?xml version="1.0" encoding="UTF-8"?>

<clientConfig version="1.1">
  <emailProvider id="my.test.com">
    <domain>test.com</domain> 
    <domain>another-domain.com</domain>
    <displayName>test.com</displayName>
    <displayShortName>Test</displayShortName>
    <incomingServer type="imap">
      <hostname>imap.test.com</hostname>
      <port>993</port>
      <socketType>SSL</socketType>
      <authentication>password-cleartext</authentication>
      <username>%EMAILADDRESS%</username>
    </incomingServer>
    <outgoingServer type="smtp">
      <hostname>smtp.test.com</hostname>
      <port>587</port>
      <socketType>STARTTLS</socketType>
      <authentication>password-cleartext</authentication>
      <username>%EMAILADDRESS%</username>
    </outgoingServer>
  </emailProvider>
</clientConfig>

Nginx config

since autoconfig does not require any SSL certificate you can setup any autoconfig request to match the same config file. For cloudron it might be better to have one config file per domain?

server {
  listen 80;
  server_name
  autoconfig.*;

  root /var/www/autoconfig.test.com;
  access_log /var/log/nginx/autoconfig.test.com/access.log;
  error_log /var/log/nginx/autoconfig.test.com/error.log warn;

  location / {
    try_files $uri $uri/;
  }
}

Aside note

in theory you should be able to not use imap.test.com and smtp.test.com, in practice though, I had trouble with thunderbird not recognizing the autoconfig without those 2 domains setup (it was 1 year ago, maybe it changed since)

Hope it helps! I'll be happy to assist implementing if interested