regarding this web.config setting:

    <!-- flag if SSL is enabled while sending an e-mail portal wide -->
      <add key="EnableSmtpSSL" value="false" />

seems that if one sets EnableSmtpSSL to true and then set their SMTP settings to connect to a local SMTP relay (say using the instructions at http://www.vsysad.com/2014/01/configure-iis-smtp-server-to-use-gmail-to-forward-messages/)

      <mailSettings>
        <smtp deliveryMethod="Network" from="xx@gmail.com">
          <network
            defaultCredentials="false"
            host="xxxxx.com"
            port="25"
            />
        </smtp>
      </mailSettings>

then the contact form WORKS, but the activation e-mail (and other email notifications) FAIL (when the local relay has been set to accept connections via Anonymous Authentication) with "System.Net.Mail.SmtpException: Server does not support secure connections."

turning that EnableSmtpSSL to false makes ALL work (both contact form and other e-mail actions)

so it seems the contact form IGNORES EnableSmtpSSL setting and uses false? Then obviously if the SMTP server requires SSL the contact form will fail?

for example I was trying to send to smtp.gmail.com using 587 port and having set that google acount (at account settings, not at gmail settings) to "allow less secure apps to connect"), but still the contact form was failing, whereas via the relay it works fine and sends to gmail (the relay takes care of it of course)