MS Outlook requests POP3 authentication several times daily

I have several users reporting that Outlook asks for their password many times every day. This has been going on for 3-4 weeks. I found the following post, but it is very old and was not resolved:

The server has also been experiencing load spikes that I have not been able to track down satisfactorily (they seem to be tied to MySQL). I thought these issues were related but have seen the Outlook dialog open when load was very low. Here are our POP3 settings:

MDA Server Information [POP3]
POP3 Version: [?] 1.03 (qmail-pop3d)
*POP3 Memory Limit: 256000000 (In Bytes)
*POP3 Connections (max): 200
*Secure POP3 Memory Limit: 256000000 (In Bytes)
*Secure POP3 Connections (max): 40

Bumping secure POP3 connections up should fix this, if it’s still a problem.