is it normal to see lots of mysql auth errors?

(ISPConfig 3.0.5.3 on top of Perfect Server Squeeze)

I'm running Courier authdaemond with DEBUG_LOGIN=1 (for some other reasons) and notice the following messages pretty frequently in syslog:

10:34:09 web13 authdaemond: received userid lookup request: vmail
10:34:09 web13 authdaemond: authmysql: trying this module
10:34:09 web13 authdaemond: SQL query: SELECT login, password, "", uid, gid, homedir, maildir, quota, "", concat('disableimap=',disableimap,',disablepop3=',disablepop3) FROM mail_user WHERE login = 'vmail'
10:34:09 web13 authdaemond: zero rows returned
10:34:09 web13 authdaemond: authmysql: REJECT - try next module
10:34:09 web13 authdaemond: FAIL, all modules rejected

It appears to be an attempt to authenticate the user vmail, instead of a regular email user.
For comparison, a successful email authentication has the sequence:
10:55:14 web13 authdaemond: received auth request, service=imap, authtype=login
10:55:14 web13 authdaemond: SQL query: SELECT login, password, "", uid, gid, homedir, maildir, quota, "", concat('disableimap=',disableimap,',disablepop3=',disablepop3) FROM mail_user WHERE login = '[email protected]'

I'm wondering if this is an indication of a misconfiguration (like maybe there should be a user vmail in the db?), or if this is a normal side effect of ispconfig in operation?

thanks,
james