The (probably postfix) smtp server you are sending mail through is having intermittent problems talking to it's auth db (probably load related - either under-resourced or being dos'ed).

So it's not your (or pmail's) issue, and there is nothing you can do (except change providers or badger them to solve it).

When sending mail, I often get this strange error. It crops up immediately. If I select the message from my "copies to self" folder and resend it, sometimes it goes OK (taking a few seconds to process), sometimes not. AFAIK, only normal Windows and Avast updates (and possibly Java) have occurred since this has started happening. These messages are typically only sent to bcc recipients. Here is a sample of the bounce message, with my email address altered for a little privacy:

Delivery has failed on the enclosed message for the following
reasons reported either by the mail delivery system on the mail
relay host or by the local TCP/IP transport module:

500 5.7.0 Unknown AUTH error -1 (Internal authentication error).

Your original mail message follows:
--------------------------------------------------------

X-PM-Identity: <Default>
From: "Paul Van Voorhees" <usernamegoeshere@verizon.net>
To:
Date: Sun, 10 Nov 2013 13:09:12 -0500
MIME-Version: 1.0
Subject: Weather Update
BCC: (Suppressed)
Message-ID: <527FCBC8.26470.71E2678@usernamegoeshere.verizon.net>
Priority: normal
X-mailer: Pegasus Mail for Windows (4.63)
Content-type: Multipart/Related; boundary="Message-Boundary-26009"

--Message-Boundary-26009
Content-type: text/html; charset=US-ASCII
Content-transfer-encoding: 7BIT
Content-description: Mail message body

Disable outbound email (av) scanning and see if it resolves the issue.

Oddly, the issue seems to have "fixed itself". Not a clue what may have changed...

Apparently even with Avast 2014.9.0.2008 outgoing mail checking turned off, it still happens occasionally. I can resend a message 3 or more times before it actually goes. But this seems to be only about every 5th message sent. The others still go through fine. Odd...