Today I learned that restarting dovecot with the systemd unit provided by debian (on buster) is not enough to reload the configuration.
systemctl restart dovecot
I had to manually run
doveadm reload
Then I realized that this is integrated in the systemd unit, using reload instead of restart:
[Unit]
Description=Dovecot IMAP/POP3 email server
Documentation=man:dovecot(1)
Documentation=http://wiki2.dovecot.org/
After=local-fs.target network-online.target
[Service]
Type=simple
ExecStart=/usr/sbin/dovecot -F
PIDFile=/var/run/dovecot/master.pid
ExecReload=/usr/bin/doveadm reload
ExecStop=/usr/bin/doveadm stop
PrivateTmp=true
NonBlocking=yes
ProtectSystem=full
ProtectHome=no
PrivateDevices=true
# You can add environment variables with e.g.:
#Environment='CORE_OUTOFMEM=1'
# If you have trouble with `Too many open files', increase
LimitNOFILE=65535
# If you want to allow the Dovecot services to produce core dumps, use:
#LimitCORE=infinity
[Install]
WantedBy=multi-user.target