The logs for e-mail sending are virtually useless

[matrixbot]

This issue has been migrated from #15170.

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• If you try to connect to a implicit TLS host with STARTTLS, the connection just hangs.
• If there's a TLS verification failure, it just retries to connect 5 times but never gives any error concerning TLS verification.
• I'm sure there are other similarly useless error cases too.

As of Synapse version v1.77.