Oddity when accessing Courier IMAP from Android

[Roberto C. Sánchez <roberto@debian.org>]

I am running a Courier IMAP server on Bookworm.

When accessing from mutt, Thunderbird, and Android (GMail app),
everything works fine. This is over SSL, with a server certificate I
have issued from my own CA and with the CA root cert distributed to all
client devices which access the IMAP server.

However, on one particular Android device, I have started to observe
that my account no longer refreshes and when I look in the server logs,
I see a LOGIN, followed by "Unexpected SSL connection shutdown" followed
immediately by a DISCONNECT.

I have seen various postings online about this, but they are mostly from
10+ years ago and have to do with things like people forgetting to trust
the certificate and things of that nature.

I'm wondering if anyone has any sort of thought on what might result in
a single device experiencing this problem when other devices are able to
access the server and the account without issue.

Regards,

-Roberto

-- 
Roberto C. Sánchez