[exim] Pulling out my hair!

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Author: Mark Elkins
Date:  
To: exim-users
Subject: [exim] Pulling out my hair!
Hi folks,

I'm pulling out my hair... I have a strange e-mail problem. I'm hoping
this community can help.

I run an ISP in South Africa. Servers are all Linux Gentoo. I use EXIM
and Courier-IMAP/POP to deliver emails to about 3000 users using about
400 different domain names. From Monday the 28th October, a few clients
- 10 so far - are sometimes getting the following....


> Begin forwarded message:
>
> *From: *Mail Delivery Subsystem <postmaster>
> *Subject: **Cannot display Unicode content*
>
> This E-mail message was determined to be Unicode-formatted
> but your E-mail reader does not support Unicode E-mail.
>
> Please use an E-mail reader that supports POP3 with UTF-8
> (see https://tools.ietf.org/html/rfc6856.html).
>
> This can also happen when the sender's E-mail program does not
> correctly format the sent message.
>
> The original message is included as a separate attachment
> so that it can be downloaded manually.

Sending a simple email works, add an emojie and it breaks.

Can anyone who has come across this enlighten me please?
I don't see the problem on my own Laptop (running Gentoo).
If the affected customer uses my webmail system - the e-mail is fine. If
they "Forward" the bad email using webmail to themselves - then the bad
email becomes good - suggesting the e-mail is rewritten.
Some customers with multiple devices find their Microsoft -or- Apple
laptop does not work, but their smart phone does work.

# exim --version
Exim version 4.92.2 #2 built 08-Oct-2019 04:01:46

Courier programs installed:-
[ebuild   R    ] net-libs/courier-unicode-2.1::gentoo  0 KiB
[ebuild   R    ] net-libs/courier-authlib-0.69.0-r1::gentoo USE="berkdb
crypt gdbm mysql pam -debug -ldap -libressl -postgres -sqlite
-static-libs" 0 KiB
[ebuild   R    ] net-mail/courier-imap-5.0.7::gentoo USE="berkdb gdbm
ipv6 -debug -fam -gnutls -libressl (-selinux) -trashquota" 0 KiB

courier-unicode-2.1 - installed 2nd March 2019
courier-authlib-0.69.0-r1 - installed 9th October 2019
courier-imap-5.0.7 - installed 28th October 2019

courier-imap looks suspicious - but I've no evidence really to say its
the culprit.

HELP!

-- 
Mark James ELKINS  -  Posix Systems - (South) Africa
mje@???        Tel: +27.128070590  Cell: +27.826010496
For fast, reliable, low cost Internet in ZA:https://ftth.posix.co.za