Dear Maintainer,

I was having certificate failures when sending mail to a sendmail
submission server and traced the issue down to sendmail not sending the
entire cert chain. The reason is that it uses
SSL_CTX_use_certificate_file() to read the file and not
SSL_CTX_use_certificate_chain_file(). Poking around the sendmail source
I noticed that this was gated behind the
_FFR_TLS_USE_CERTIFICATE_CHAIN_FILE define. I made a patch to
debian/configure.ac to add that, rebuilt the package, and verified that
it now sends the whole cert chain.

I've attached the patch.

I think it's an important feature--nowadays most "real" SSL certs have
intermediates in them, especially in the LetsEncrypt/ACME space, and
sendmail will not produce a validating cert chain unless this option is
enabled.

Thanks,
   David