Ok, been going around and around with this.

I've got a VPS running Plesk 8.6 with the QMail from Plesk installed on CentOS 5.2

I have configured mail to "enable message submission", relaying has authorization required for both POP and SMTP with a 20 minute lock time for the POP, using Dr. Web antivirus, both DomainKeys options are selected for incoming and outgoing, SPF Spam protection is on, so is the blackhole list (using sbl.spamhaus.org), and only full names are allowed.

When I telnet into the server on port 25 or 587, I am able to send mail fine with no authorization set (however all "open relay" online test say that I am secure). But when I do an "auth login" and go through the process using Base64 username and passwords, I get a "535 auth failure". As found in Parallels KB#1390, I have done the mchk --without-spam with no problems. Checking that SMTP (KB #1387) is fine and it does check out.

Everything seems to be working perfectly except that I can not do smtp authentication. Could I have some privileges off somewhere? I haven't changed anything really in command line that would affect QMail. All changes have been within Plesk.