[Bradford] Email server config problems

[Devo Too]

Hi Folks,

Any postfix/dovecot gurus out there? This tutorial 
https://thomas-leister.de/en/mailserver-debian-stretch/ helped create 
the server this email is coming from, on Debian Stretch. It was done 
successfully with few if any problems on the way.

It's time to move from that old creaking server onto one with Debian 
Buster. So the email server is built is built from the same tutorial but 
it won't allow T'bird to log in.

After a lot of reading of man pages, web pages and config files, the 
nearest I have come to anything which may offer a clue (although I do 
suspect it's an error in an as yet unidentified postfix or dovecot 
config file) is a Debian site discussion where the person who had a 
similar, but not the same, fault cured it temporarily by knocking 
"reject" off the ends of a block of lines in postfix's main.cnf file. 
But if my understanding is correct, each line will only be followed by 
the system as far as the next implementable command. So having reject on 
the end should be after it could be operated (postfix should have done 
something else and moved on before it got that far).

However, there is an MUA block where each line ends in "reject". IIUC, 
T'bird is an MUA (Mail User Agent). Could the word be deleted in each 
instance, replaced with something else, or is that likely to be a red 
herring so leave it alone?

Any other suggestions for tracing where the fault may lie? Any 
suggestions for an alternative mail client which could be run to see if 
the fault is with T'bird itself? Not Mutt or Pine though. That would 
just be another (to me) incomprehensible manual to read

TIA,

Mike