Está vendo muitas entradas como estas no seu log do vpopmail ?
Mar 27 11:04:59 neo vpopmail[2317]: vchkpw-pop3: vpopmail user not found katie@:68.126.100.142
Mar 27 11:05:00 neo vpopmail[2322]: vchkpw-pop3: vpopmail user not found katrina@:68.126.100.142
Mar 27 11:05:00 neo vpopmail[2325]: vchkpw-pop3: vpopmail user not found katie@:68.126.100.142
Mar 27 11:05:01 neo vpopmail[2329]: vchkpw-pop3: vpopmail user not found katrina@:68.126.100.142
Mar 27 11:05:02 neo vpopmail[2339]: vchkpw-pop3: vpopmail user not found katrina@:68.126.100.142
Mar 27 11:05:03 neo vpopmail[2373]: vchkpw-pop3: vpopmail user not found katrina@:68.126.100.142
Mar 27 11:05:04 neo vpopmail[2381]: vchkpw-pop3: vpopmail user not found katrina@:68.126.100.142
Mar 27 11:05:05 neo vpopmail[2385]: vchkpw-pop3: vpopmail user not found katrina@:68.126.100.142
Mar 27 11:05:09 neo vpopmail[2389]: vchkpw-pop3: vpopmail user not found katrina@:68.126.100.142
Mar 27 11:05:11 neo vpopmail[2393]: vchkpw-pop3: vpopmail user not found katie@:68.126.100.142
Solução para isto, do modo mais fácil. Abra o seu arquivo tcp.smtp :
e adicione
68.126.100.142:deny
Dê um rebuild no arquivo ( em geral, qmailctl cdb ) e fique livre destas pragas :-)