https://blog.andreev.it/?p=1975
I have set up postfix and dovecot on both centos and freebsd using this person's blog. While you are using Debian, you might find the test procedures in this blog useful. You can test the set up without using an email client. That is the testing gets around client configuration issues because no client is used in testing. 

This is a stick shift email installation. No fancy scripting. Every step is tested. You don't go to the next step until the one you are testing works. 

You can probably adapt this for Debian. Personally I would rather used centos for a server. It is drama free but never cutting edge. I like cutting edge on the desktop but not on the server. 

From: nils@bdevgw.de
Sent: July 19, 2020 11:54 AM
To: dovecot@dovecot.org
Subject: Re: I need some help with my Dovecot and Postfix configs - I'm unable to log in on my mail server

Autocinfiguration is fine, my problem is that once everything is (auto)configurated (correctly, checked this) that the server doesn't accept my login request.
STARTTLS is correct, ports are correct etc. My mail is correct, my password is correct (tried with copy paste) and also with name as username and name@domain aswell (name was also copy pasted).

On 19/07/2020 12:43, Bernardo Reino wrote:
On Sun, 19 Jul 2020, Nils wrote:

This is what my server logs (mail.info,         mail.warn) tell me:       
        root@bgrsld-mail0:~# tail /var/log/mail.info       
        Jul 17 18:22:08 bgrsld-mail0 postfix/submission/smtpd[8472]:         improper command pipelining after EHLO from         unknown[192.168.2.110]: QUIT\r\n       
        Jul 17 18:22:08 bgrsld-mail0 postfix/submission/smtpd[8465]:         disconnect from unknown[192.168.2.110] ehlo=1 quit=1 commands=2       
[...]

Thunderbird, for some reason, violates the SMTP standard when attempting autoconfiguration. It sends multiple commands ("pipelining") without postfix having announced that it's OK to do so.

You can either do the configuration manually (when Thunderbird fails, I think you can still go to "manual" or "advanced" or whatever button to continue with the configuration), or you could, at least temporarily, disable postscreen (which is the only complaining -- rightly -- about the improper pipelining), and then enable it again once you have configured your account.

You can also read:
https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration

and set-up the necessary XML file at your server so that Thunderbird can pickup the settings automatically. I've done this for one server, but don't have the details anymore in my head. The link above should explain that all though.

Good luck!
Bernardo