It's not directly related to emails, it's a generic error when connecting to any external host via SSL/HTTPS.

I.e. the following fails:

using (var client = new SmtpClient
   {
       EnableSsl = true,
       Port = AppContainer.Configuration["Umbraco:CMS:Global:Smtp:Port"].ToInt(),
       Host = AppContainer.Configuration["Umbraco:CMS:Global:Smtp:Host"],
       Credentials = new NetworkCredential(AppContainer.Configuration["umbraco:CMS:Global:Smtp:Username"],
           AppContainer.Configuration["umbraco:CMS:Global:Smtp:Password"])
   })

But this also fails with same error:

var response = new WebClient().DownloadString("https://url-to-som-external-site");