Thunderbird autoconfig does not respect the ISP's hints

https://boum.org/.well-known/autoconfig/mail/config-v1.1.xml has:

<code class="html">
    <outgoingServer type="smtp">
      <hostname>boum.org</hostname>
      <port>587</port>
      <socketType>STARTTLS</socketType>
      <authentication>password-cleartext</authentication>
      <username>%EMAILADDRESS%</username>
    </outgoingServer>
    <outgoingServer type="smtp">
      <hostname>boum.org</hostname>
      <port>465</port>
      <socketType>SSL</socketType>
      <authentication>password-cleartext</authentication>
      <username>%EMAILADDRESS%</username>
    </outgoingServer>

… but with thunderbird 60.0~b10-1~deb9u1.0tails1, the autoconfig wizard will select SMTP on port 25 with STARTTLS as the outgoing server when configuring a boum.org email account. I did not check how earlier versions (until Tails 3.8 inclusive) behave. If they behave differently, then maybe the version of the patch set that was heavily reworked for Thunderbird 60 has a regression.

Parent Task: #6156

Original created by @intrigeri on 15790 (Redmine)