Mac OS Mail: An SSL error has occurred and a secure connection to the server cannot be made.

I get the following message from time to time when I try to send an email:

An SSL error has occurred and a secure connection to the server cannot be made.

I check settings, logs, etc. and check against recommended settings from my ISP and everything is as it should be. Also, the message is random so everything is fine for most of the time, then it plays up.

Similarly, sometimes incoming mail takes ages to show up in my inbox, which is really annoying when email is used to verify registration for an on–line service or account.

I use a VPN and typically find that quitting Mail, changing the VPN server, then restarting Mail seems to fix things. But I don't understand why these issues occur in the first place.