None wrote:

sendmail[23425]: STARTTLS=server, error: accept failed=0, SSL_error=1, errno=0, retry=-1, relay=m68-112.mailgun.net [166.78.68.112]

Are there more errors for the same process?
Which LogLevel do you use?

mailgun is a #%$&@& wrt TLS: they configure weird requirements
and simply hang up if they don't "like" something.

sendmail[76688]: STARTTLS: write error=syscall error (-1), errno=104, get_error=error:00000000:lib(0):func(0):reason(0), retry=99, ssl_err=5

You might want to update your sendmail version so you don't just
get errno= but also the textual representation.

On Linux 104 is:
/usr/include/asm-generic/errno.h:#define ECONNRESET 104 /* Connection reset by peer */

2. I assume that in the smtp standard the option to fallback to the
plain connection is still mandatory?

Was it ever?

--
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reply to top-postings which include a full copy of the previous
article(s) at the end because it's annoying, shows that the poster
is too lazy to trim his article, and it's wasting the time of all readers.

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