Event Type: Error
Event Source: POP3SVC
Event Category: General 
Event ID: 1051
Date: 5/17/2013
Time: 9:40:21 AM
User: N/A
Computer: IPCSMAIL
Description:
Unexpected error condition: call to function CEncryptCtx::CheckServerCert() resulted in error code 0x800cc801. 


This happens, when you removed check mark for Basic authentication ( password send in clear text ) in access control for POP3 virtual server properties and mail client not supporting this. 

Resolution : 

Right click on SMTP virtual server under protocols > Access > Authentication > check mark Basic authentication ( password send in clear text )