I recently encountered a problem in accessing my POP-3 served e-mails via KMail. All accounts were originally set without encryption, then last week I couldn't access any of the accounts, the system informing me that there was a problem with the ssl certificate. Having checked with my ISP that they had not made any changes in this area, I investigated my KMail configuration, and found that all accounts had been set with SSL/TLS encryption, and directed to Port 995, rather than the originally set Port 110. I am using Version 4.13.3 on a PCLinuxOS platform which sits behind a router firewall and has a fully updated ClamAV anti-virus package. I have not noticed any recent upgrades to KMail, and I am at a loss to see how I could have inadvertently re-set all the accounts to require encryption. If anybody can clarify this behaviour, I would be very grateful.

I haven't seen this behavior. I'm not sure how it could change, either, unless something applied changes to your configuration (but I doubt it).