Uverse uses port 995 with SSL. You'll need to add SSL to unraid or use gmail.There you go. Try using port 995. (and use TLS = YES) certainly worth a try.
I have no other answers... I don't use your e-mail provider. I cannot test, since most will not accept connections from other than their own subscribers. (something about rejecting SPAM sent from bad-guys)
Google is your friend. Others might be using ssmtp with the same provider and are going through the same configuration steps.
Some e-mail providers are very picky about what they will accept.
Perhaps I need to back up and make sure I'm editing the correct files. Which file(s) should I be editing to make the smtp port changes as well as the TLS change? UnMenu reports the following for ssmtp:
Installed, but version is different.
Current version='2.60.4' expected '2.62'
When I try to install ver 2.62 I get an error that the C compiler is not installed and the notes in that package say it's unlikely that I should need to install it.
The C compiler has not been installed. ssmtp compile not possible
ssmtp_2.62.orig.tar.gz apparently did not install properly, /usr/local/etc/ssmtp/ssmtp.conf does not exist.
Should I install the compiler and if so, do I need swap space? I have 2GB of RAM on the server.