[exim] SMTP timeouts

I have started seeing odd timeouts from Exim when talking to Goolge
gmail, email path:

    MTA (Thunderbird/Win 10) --> mail.tubby.org (Linux/Exim) -->
relay1.thorcom.net (Linux/Exim) --> Gmail

My MTA is behind my firewall, it sends SMTP to my public mail server,
which relays (smart host) via relay1.thorcom.net.

Relay1.thorcom.net is Devuan 4 Chimaera with Exim 4.94.2 built from source:

root@relay1:/var/log/exim# exim --version
Exim version 4.94.2 #2 built 04-Jan-2022 14:20:39
Copyright (c) University of Cambridge, 1995 - 2018
(c) The Exim Maintainers and contributors in ACKNOWLEDGMENTS file, 2007
- 2018
Berkeley DB: Berkeley DB 5.3.28: (September  9, 2013)
Support for: crypteq iconv() IPv6 Perl OpenSSL move_frozen_messages
Content_Scanning DKIM DNSSEC Event OCSP PIPE_CONNECT PRDR SPF
TCP_Fast_Open Experimental_SRS
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch dbm dbmjz
dbmnz dnsdb mysql passwd
Authenticators: plaintext
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir/mbx autoreply pipe smtp
Malware: f-protd f-prot6d drweb aveserver fsecure kavdaemon sophie clamd
mksd avast sock cmdline
Fixed never_users: 0
Configure owner: 0:0
Size of off_t: 8
Configuration file is /etc/exim/exim.conf


Smaller text-only emails to Gmail users work fine. Attach a full HD
image (1920 x 1080 Jpeg) and the message ends up being around 980K in
size, when I attempt to send it it stalls on relay1.thorcom.net.

Logs on relay1.thorcom.net show incoming from my server (mail.tubby.org)
just fine:

2022-03-13 19:13:03 CONNECT: Accepting connection from:
2a02:8010:7010::6700 - not blocked by any RBL
2022-03-13 19:13:03 HELO: Accepted HELO/EHLO mail.tubby.org from remote
host: 2a02:8010:7010::6700
2022-03-13 19:13:03 CRYPTO: Client 2a02:8010:7010::6700:47210 issued
STARTTLS
2022-03-13 19:13:03 HELO: Client 2a02:8010:7010::6700:47210 using
SSL/TLS cipher: TLS1.3:TLS_AES_256_GCM_SHA384:256
2022-03-13 19:13:03 HELO: Accepted HELO/EHLO mail.tubby.org from remote
host: 2a02:8010:7010::6700
2022-03-13 19:13:03 MAIL: SPF Result=pass (tubby.org / (mail.tubby.org)
[2a02:8010:7010::6700])
2022-03-13 19:13:03 H=(mail.tubby.org) [2a02:8010:7010::6700] Warning:
MAIL: TLS-STATUS Sender domain=tubby.org Host=(mail.tubby.org)
[2a02:8010:7010::6700] using TLS cipher=TLS1.3:TLS_AES_256_GCM_SHA384:256
2022-03-13 19:13:04 1nTTeJ-0001Tn-AC H=(mail.tubby.org)
[2a02:8010:7010::6700] Warning: DKIM START: domain=tubby.org
possible_signer=tubby.org status=pass
2022-03-13 19:13:04 1nTTeJ-0001Tn-AC H=(mail.tubby.org)
[2a02:8010:7010::6700] Warning: MIME: Type=multipart/mixed Size=931
2022-03-13 19:13:04 1nTTeJ-0001Tn-AC H=(mail.tubby.org)
[2a02:8010:7010::6700] Warning: MIME: Type=text/plain Size=1
2022-03-13 19:13:04 1nTTeJ-0001Tn-AC H=(mail.tubby.org)
[2a02:8010:7010::6700] Warning: MIME: Type=image/jpeg Size=688
(Filename=IMG_0612.jpg)
2022-03-13 19:13:05 1nTTeJ-0001Tn-AC <= mike@???
H=(mail.tubby.org) [2a02:8010:7010::6700] P=esmtps
X=TLS1.3:TLS_AES_256_GCM_SHA384:256 CV=no S=954453 DKIM=tubby.org
id=4b53323e-feba-2b43-11df-adadac2ff6d8@??? T="Michelle 2"

Exim logs the incoming message "<=", the message ends up on the spool
and is visible with mailq.

There are no obvious signs that Exim is trying to complete the delivery,
i.e. no "=>" and after a period (several minutes) it logs:

2022-03-13 19:47:53 1nTTGO-0001Jw-Tr H=alt2.gmail-smtp-in.l.google.com
[2a00:1450:4025:c03::1a]: SMTP timeout after sending data block (476909
bytes written): Connection timed out


So, what's occurring here?  Does google/gmail have indigestion?


Mike