I was again bit by this error which appears to be caused by Norton security software.
I ran into this error once before when a client reported to me that his computer was popping up warning from some Norton security software whenever he tried to send an email. I tried to figure out what was going on, but in the end had to switch to port 587.
The latest report was also able to be fixed by switching to port 587, but the client didn't inform me that Norton was issuing warnings at first. I did some digging and finally found the "too many errors after RSET" error (from Postfix), but was unable to diagnose it until he mentioned Norton.
While I was at it, I raised the error limit which causes a disconnect when the client sends too many junk commands:
smtpd_junk_command_limit = 1000