I did try that, but it makes no difference - note the error message in the Dovecot log file, which actually shows the '*' verbatim (or the '?', if you use that), so the special character actually goes through to doveadm (I'm not sure how!)

I'm trying to get a list of all dovecot users, on a simple test setup on
my local network (which "works", in that I can get IMAP mails from dovecot):

passdb {
   driver = passwd-file
   args = /var/mail/vhosts/%d/shadow
}
userdb {
   driver = passwd-file
   args = /var/mail/vhosts/%d/passwd
}

The command 'doveadm user -u foo@example.com' correctly shows that user.
However, any attempt to wildcard the user fails:

# doveadm user -u *@example.com
Error: auth-master: userdb list: User listing returned failure
Fatal: user listing failed

The log file shows

 quotes? shell expands *