After trying over hours getting ssmtp's starttls feature working using the provided default config I gave up and downloaded the source tarball. Provided with the source there is also a file called "TLS". It contains the description of the SSL/TLS-specific enhancements and *all* configuration settings. I suggest to add at least the (commented out) "UseSTARTTLS=" configuration setting to ssmtp-ebuild's default ssmtp.conf file. Attached you'll find a small patch of the ebuild-shipped ssmtp.conf
Created attachment 84946 [details, diff] added the UseSTARTTLS option and changed two comments to make SSL/TLS/STARTTLS configuration more obvious...
We tend to ship whatever upstream ships if it is acceptable. And the ssmtp config file is good enough. The TLS documentation file is also installed by the ebuild. Cheers, Ferdy