sSMTP is a simple program that punts mail over to a mailhub. I use this on a few Ubuntu hosts. I recently deployed a Raspberry Pi running Raspian that I wanted to enable mail alerting with. I quickly found that it could not send e-mails with sSMTP. At first, I assumed it was my configuration but I investigated and found an issue with TLS handshakes.
sSMTP has a known (and fixed) problem with TLS 1.3 handshakes. According to the changelog, this was fixed in August 2019.
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