Major upgrade of mail server

[Shawn Heisey]

I have a mail server in AWS that is currently running Ubuntu 18.  Every 
time I log in, I am reminded that I can upgrade to Ubuntu 20.

On Ubuntu 18, the dovecot version is 3.3.0-1ubuntu0.3.  On Ubuntu 20, it 
is 2.3.7.2-1ubuntu3.  Many other packages, probably including the mysql 
server, would also be upgraded.

Dovecot and Postfix use a postfixadmin database in mysql for users, and 
postfix is using dovecot-lda to deliver mail.  I am using managesieve 
from dovecot on roundcube webmail.  As far as I know, my own user is the 
only one with sieve scripts actually in use ... and I have a LOT of 
filters/folders for various mailing lists.

I've been a little bit terrified of doing an upgrade, because I do have 
a couple of people using my mail server for real work email and I don't 
want to disrupt them.

I'm writing today to find out what are the likely pain points I might 
encounter when doing this kind of major upgrade, and if there is any 
helpful information that can help me get through those problems.  I'm 
hoping that it will go smoothly and everything just works.

Here's the doveconf -n output:
https://apaste.info/FUgF

If I have been silly enough to include sensitive data from the config, I 
would appreciate a heads up so I know what passwords to change.  I did a 
quick glance and didn't see anything.

Thanks,
Shawn