How does dovecot determine users from /etc/passwd?

[Philip Colmer]

We're using dovecot on a help desk ticketing system. The system
doesn't perform any cleanup of IMAP mailboxes so we have a regular
cronjob doing this:

/usr/bin/doveadm flags add -A '\Deleted' SEEN SENTBEFORE 12w &&
/usr/bin/doveadm expunge -A DELETED MAILBOX '*'

Recently, doveadm has been reporting this:

doveadm(nobody): Error: User initialization failed: Namespace '':
mkdir(/nonexistent/mail) failed: Permission denied (euid=65534(nobody)
egid=65534(nogroup))
doveadm(nobody): Error: User init failed
doveadm(DEFAULT-CW-USER): Info: User no longer exists, skipping

Indeed, using

doveadm user "*"

does include "nobody" in the list but I can't understand why. The mail
group in /etc/group does not include "nobody" and there are users in
/etc/passwd with similar attributes to "nobody" that do not appear in
the list of users from doveadm.

What can I do to tell doveadm to forget about "nobody"?

Thanks.

Philip