I have seen this time and again and the forums worldwide are filled with wrong answers. Microsoft explains that it may be a corrupt file, to Detect and Repair, start Outlook in safe mode, etc. While this may be applicable to some circumstances, more often than not, it may be a simple solution. Some are worried that it is a problem with the wonderful program ZoneAlarm, that I wouldn’t know.
Suggestion:Some ISP’s make you use pop.YOURSITE.com and smtp.YOURSITE.com.
Try mail.YOURSITE.com (for your pop3 setting) AND mail.YOURSITE.com (for your smtp setting).