Last week my send/SMTP functionality stopped working in eM Client. Since I am running Windows 7 Pro SP1, I thought it may have something to do with the TLS changes that are happening with Microsoft so I followed all the necessary steps in the following article: Update to enable TLS 1.1 and TLS 1.2 as default secure protocols in WinHTTP in Windows
In addition, I confirmed SMTP is working fine in Thunderbird. I have tried numerous ports and all the security policy (SSL/TLS) options in eM Client. I also un-installed/re-installed eM Client and I am on the latest version. Still a no go.