As I said in my post I have tried all the 4 possible options in the smtp security tab including Force usage of SSL/TLS and none of them work. Been struggling with this for some days and getting mighty frustrated. There also seems to have been many posts on here about the same problem.
However thanks for the info. If you can confirm that this actually works for you then I will have to look elsewhere for the problem solution.
I always use port 587 for SMTP, even if the default setup selects 465. The above screenshot is for my GMX.com account, but the same works for a number of others that I use.
But this may have to do with your email provider, or even your anti-virus/firewall application. Maybe temporarily disable the anti-virus, then try again. If you name the provider, maybe someone using them could comment.
If that still does not work, go to Menu > Operations and look for any SMTP errors in the Log tab.
I have now managed to find a setting which does work with Port 587 in em Client when using my ISP’s own mail servers however I still do have a problem with Thunderbird settings even if I apply the same settings as in emClient. Still working on it.