eM Client not syncing - SMTP settings temporary fix

E-mails have not been syncing for a while now. Re-installing eM Client did nothing.
However, changing the SMTP security policy from ‘Force usage of SSL/TTS’ to ‘Use SSL/TTS on special port (legacy)’ temporarily fixed the synchronization issue - e-mails were now being received with no problem. _Sending_ e-mails no longer worked however, until the setting was reverted to ‘Force usage of SSL/TTS’.
Now, even by switching between the two, synchronization is no longer working.

For me, eM Client has run its course. I don’t have the time to be constantly tweaking my e-mail program to get it working. Perhaps this observation helps someone else! 

The port and policy combinations are important. Random mixing does not work.

Port 587 = Force usage of SSL/TLS 
Port 25 = Use SSL/TLS if available 
Port 465 = Use SSL/TLS on special port (legacy) 

Port 993 = Use SSL/TLS on special port (legacy) 
Port 143 = Use SSL/TLS if available 

Port 995 = Use SSL/TLS on special port (legacy) 
Port 110 = Use SSL/TLS if available

Something else that can affect connection is your anti-virus/firewall or VPN application, so if the above combinations still don’t work, try disabling those applications and see if there is any difference.

You can also look in Menu > Tools > Operations for any errors in the Log tab. That may give a better insight into what is going wrong.