ssmtp: Cannot open smtp.gmail.com:587

There are a few small typos in your config. You have written:

UseSTARTCTLS=YES
        ^

(I have marked the error) where you should have written:

UseSTARTTLS=YES

And finally the path for the certificates in Trusty should be written not as this:

TLS_CA_File=/etc/pki/tls/certs/ca-bundle.crt

but more correctly as:

TLS_CA_File=/etc/ssl/certs/ca-certificates.crt

These corrections should be enough to get you going :)

References:

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