Debian Upgrade Stretch To Buster

This article contains all problems that I encountered when upgrading Debian from 9/Stretch to 10/Buster. Since Debian 9 is already out of standard support, this should not affect a lot of people.

authmysqlrc

"authdaemond: marker line not found in /etc/courier/authmysqlrc" in /var/log/mail.err

Solution: Add these lines at the end of the file:

##NAME: MARKER:0
#
# Do not remove this section from this configuration file. This section
# must be present at the end of this file.

Credits: https://sources.debian.org/src/courier-authlib/0.69.0-2/authmysqlrc/ (found by googling "authmysqlrc example")

"authdaemond: authmysql: cannot parse the MYSQL_OPT setting" in /var/log/mail.err

Add

MYSQL_OPT		0

according to above source.