'PLAIN encryption needed to use mechanism' error

[Blake Hudson]

I recently setup a new server and everything tested well. However, once 
in production I am seeing errors like the following:

pop3PRTC[20896]: badlogin: [204.x.x.x] PLAIN encryption needed to use 
mechanism


I wasn't aware that POP utilized other mechanisms? I can login just fine 
with telnet and tbird, and cannot replicate this error myself. Any ideas?

--Blake