[Dovecot] doveadm -A stops processing at first uid<first_valid_uid

[Joseph Tam]

I would like to run various doveadm commands that involves all (mail) users like

 	doveadm expunge -A mailbox Trash savedbefore 30d

but any doveadm command that uses "-A" to iterate through all users will
stop processing at the first account with UID<first_valid_uid.

 	doveadm(sysdaemon): Error: user sysdaemon: Couldn't drop
 		privileges: Mail access for users with GID 5551 not permitted
 		(see first_valid_gid in config file, gid from userdb lookup).
 	doveadm(sysdaemon): Error: User init failed
 	doveadm: Error: Failed to iterate through some users

However, these accounts are system accounts (locked password, no shell)
and are in userdb to provide UID<->name mapping for utilities like ls,
chown, etc.

There are various workaround like iterating manually, or setting
first_valid_uid to zero, or even reordering users to put all system
accounts at the end, but is there a better way to do this?

Joseph Tam <jtam.home at gmail.com>