We have <add key="SmtpEnableSsl" value="true" /> set in web.config (not sure it's actually being honored, though). If you aren't modifying the message then feel free to close this ticket and we'll chalk it up to whatever server ScreenConnect is trying to use to do SMTP.
If you'd like us to open a separate support issue whereby we attempt to troubleshoot the root issue on our end (mail server availability, no doubt) then that'd be fine. This bug entry was opened as ScreenConnect failing to display complete error codes where space is available (and there is plenty of space on the login page) should rightly be considered a bug, in my opinion.
Anyways, we have ScreenConnect set to use the domain mail server as we couldn't get Gmail's SMTP relay working despite both whitelisting our IP in Gmail and trying various permutations on the SMTP/SSL settings (we received an authentication required message in every instance despite providing valid credentials)
Customer support service by UserEcho