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

And this error messages:
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

1. I am wondering if there is a clear html page where I can lookup what
these errors are.

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

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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