When the SMTP courier gets an error on STARTTLS (either because the
command itself failed, or because there was a low-level TLS negotiation
error), today we just fail that attempt.
This can cause messages to never be delivered if the underlying reason
is a server misconfiguration (e.g. a server certificate that Go cannot
parse). This is quite rare in practice, but it can happen.
To prevent this situation, this patch adds logic so that the SMTP
courier retries over plaintext when STARTTLS fails.
This is still subject to security level checks, so this type of failures
cannot be used to downgrade connections to domains we successfully
established a TLS connection previously.
Note that certificate validation issues are NOT included in this
type of failure, so they will not trigger a retry. The certificate
validation handling is unchanged by this patch.
ioutil package was deprecated in Go 1.16, replace all uses with their
respective replacements.
This patch was generated with a combination of `gofmt -r`, `eg`, and
manually (for `ioutil.ReadDir`).
