DKIM not detected when Thunderbird used

Question

I have installed Postfix on my Ubuntu server and configured DKIM. I have tested DKIM on https://www.appmaildev.com/en/dkim and the results were good. However, I am confused as to why DKIM is only detected when I send it directly through the server command line with the syntax echo "hello" | sendmail account@gmail.com, but DKIM is not detected when I send emails using Thunderbird. What am I missing here?

this is the raw message if i sent it via command line

Received-SPF: pass (google.com: domain of myaccount@coderproject.my.id designates 103.56.149.104 as permitted sender) client-ip=103.56.149.104;
Subject: test
To: mygmailaccount@gmail.com
Content-Type: image/jpeg
Content-Disposition: attachment; filename=COBA.jpg
Content-Transfer-Encoding: base64
Message-Id: <20230320204600.4D63463641@mail.coderproject.my.id>
Date: Mon, 20 Mar 2023 20:46:00 +0000 (UTC)
From: myaccount Bahar <myaccount@coderproject.my.id>

this is the raw message if i sent it via thunderbird email client

Received-SPF: pass (google.com: domain of myaccount@coderproject.my.id designates 103.56.149.104 as permitted sender) client-ip=103.56.149.104;
Content-Type: multipart/alternative; boundary="------------6D70L2ROq60OF2n6AabTQzgl"
Message-ID: <aeb4f54c-89ea-a6aa-25d2-13bed81815ba@coderproject.my.id>
Date: Tue, 21 Mar 2023 03:55:02 +0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0
Content-Language: en-US
To: mygmailaccount@gmail.com
From: myaccount Bahar <myaccount@coderproject.my.id>
Subject: Test Pesan

--------------6D70L2ROq60OF2n6AabTQzgl
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

Test Pesan

Test Pesan Attachment Gambar
--------------6D70L2ROq60OF2n6AabTQzgl
Content-Type: multipart/related; boundary="------------S6Ey91uKS7BRtwsdBF6peNf4"

--------------S6Ey91uKS7BRtwsdBF6peNf4
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit

<html>
  <head>

    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
  </head>
  <body>
    <p>Test Pesan</p>
    <img moz-do-not-send="false"
      src="cid:part1.g3cj6xzE.YjwD0uxI@coderproject.my.id" alt="Test
      Pesan Attachment Gambar" width="1024" height="768">
  </body>
</html>
--------------S6Ey91uKS7BRtwsdBF6peNf4
Content-Type: image/jpeg; name="04.jpg"
Content-Disposition: inline; filename="04.jpg"
Content-Id: <part1.g3cj6xzE.YjwD0uxI@coderproject.my.id>
Content-Transfer-Encoding: base64


--------------S6Ey91uKS7BRtwsdBF6peNf4--
--------------6D70L2ROq60OF2n6AabTQzgl--

This is not really a security question. It is more a question on how to setup DKIM and your mail clients properly so that it works in all your use cases. Unfortunately your current description is missing relevant details, specifically how Thunderbird is configured regarding sending mail in contrast to "sendmail ...". — Steffen Ullrich, Mar 17 at 18:06
If you installed postfix on the server, then it's possible that you still have some other MTA running on the server, and /usr/sbin/sendmail is routing messages through that MTA. Try sending a message via the command line to your gmail address. Then, in gmail, take a look at the headers of the message. Are you seeing that it was sent via postfix, or some other MTA? — mti2935, Mar 17 at 18:08
hi i am really sorry for my late response i still need to figure it out why when i send an email via command line it show dkim but when i send it using email client like thunderbird or email client windows 10 it not show dkim. When i take alook at the header i saw that it was sent via postfix. — Ikhsan MPHG, Mar 19 at 2:26
@SteffenUllrich how the correct way to configured Thunderbird so when i send email via Thunderbird it also give a dkim inside the mail that i send ? — Ikhsan MPHG, Mar 19 at 2:28
@SteffenUllrich when i used dkim checker like this website dmarcian.com/dkim-inspector the page show that we need to input our dkim selector, so the checker can check our dkim available or not, what i think about email client like Thunderbird or Email Client Windows 10 maybe just same, we also need to set our dkim selector inside email client that we used, is it correct ? — Ikhsan MPHG, Mar 19 at 2:33

Ralf Geschke · Accepted Answer · 2023-03-25 15:53:15Z

I had similar problems by using Thunderbird with my mail server. The situation was as follows:

Own mail server configured with ISPconfig and multiple domains.
I used one main mail address, but defined some alias addresses in Thunderbird, so I could use different sender addresses.
For outgoing mail I defined only the username (mail address) of the main account, so the SMTP login was handled with the same data for every mail address.

In my case, this was exactly what caused problems.

I had to change the outgoing SMTP server for every identity used in Thunderbird, so the SMTP login matched with the sender address.

A quick example:

FQDN of mail server: mail.example.org
each domain has its own MX records in DNS, so mx.example.com and mx.example.net have the same IP address as mail.example.org
Main user account on the mail server: john@example.org
Thunderbird:
- Default identity: name: John Doe, email: john@example.com
- Alias identity: name: John Doe, email: john@example.net

The first time I tried this, I used the account "john@example.org" for outgoing email and the SMTP server "mail.example.org" because this was the real FQDN, and the account worked fine in the past. But in this case DKIM failed.

The solution was to define different SMTP server options for each address:

identity john@example.com: SMTP server: mx.example.com, username: john@example.com (and its password)
identity john@example.net: SMTP server: mx.example.net, username: john@example.net (and its own password)

Then select the correct outgoing SMTP server in the Thunderbird account settings.

You can test the configuration with https://www.mail-tester.com/?lang=en (note - there are only three free checks per day).

Maybe this will help. In my case it was actually not a problem with the Postfix or SMTP server settings, but just with using the correct address settings when sending mail.

Kind regards, Ralf

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