Errors ALL the time.  connection issues ALL the time.  Only on this program. [CalDAV] An attempt to connect to [your email] failed.  blah blah blah…do you want to check…check what…and it eventually goes through…but it happens a LOT.

Check your anti-virus application. This kind of software often blocks applications or their ports from accessing the Internet. Sometimes when everything was fine, and now it is not, it might be because of a software update which has changed the settings. Temporarily disabling the anti-virus application will determine if it is the cause. If it is, you will need to enable specific applications/ports in that software to allow eM Client to pass through before you re-enable it again. You are only disabling the application temporarily to test sending and receiving email, so don’t worry about security. Just don’t open any unknown files or install any applications while it is disabled.

If you have either a VPN or proxy running, disable them to determine if they are the cause.

Go to Menu > Tools > Accounts and make sure you are using the correct server settings. Providers often recommend specific settings, so go with those first. If that does not help, choose from one of the correct combinations below. The underlined ones are the best option for security if your server supports them. SMTP port 465 was reassigned and should no longer be used for SMTP, though many mail providers still recommend and use it. Use it as a last resort.

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

The combination is important. Having the incorrect policy can result in your password being rejected, or the connection being dropped. You provider may have slightly different names for the security policy, so go with whatever port they recommend, and choose the correct policy from the list above.