[exim] Issue with Exim on an IPv6-only host

Hello there,

Exim is configured to listen only on local interfaces and deliver
_all_ mail to a remote SMTP server requiring TLS and authentication,
with all local mail being sent to single email address.

Here are the four most pertinent lines written to mainlog
(abbreviated, redacted and wrapped):

--8<---------------cut here---------------start------------->8---

H=mail.….com […] TLS error on connection (recv): The TLS connection
was non-properly terminated.

H=mail.….com […] TLS error on connection (send): The specified
session has been invalidated for some reason.

** […] R=all_via_fast_smtp_server T=fast_smtp_server […]: SMTP error
from remote mail server after pipelined MAIL FROM:<[…]> SIZE=1537:
530 5.7.1 Authentication required DT=1m

--8<---------------cuthere---------------end--------------->8---

swaks and openssl's s_client have no problem negotiating TLS and
testing with each shows that there's nothing wrong with the
authentication credentials provided in the conf file.

An effectively identical conf file employed on an IPv4-only host
(running the same version of Debian) also encounters no such problem.

The virtual host provider operates a NAT64/DNS64 service for IPv6-only
hosts which allows only _outbound_ connections to IPv4 only services.
Might this explain why the TLS connection is non-properly terminated?

Yet, swaks and s_client negotiate TLS without a problem. I'm stumped.

A redacted exim4.conf is attached.

Any help/tips/suggestions very much appreciated.

Sebastian

P.S. Exim version 4.94.2 #2 built 13-Jul-2021 […].