On 14. Oct 2020, at 14.20, @lbutlr <kremels@kreme.com> wrote:

On 14 Oct 2020, at 04:24, Sami Ketola <sami@ketola.io> wrote:

On 14. Oct 2020, at 12.51, Victor Sudakov <vas@sibptus.ru> wrote:

Those Mac clients who do not use nested folders do not seem to be
affected. That is, I've already suggested to the Mac Mail user that he
start Thunderbird, move all his mailboxes into the IMAP root folder, and
then go on using Mac Mail. I hope that's what he has done.

There is a bug in Mail.app that it always assumes / as hierarchy separator and 
completely ignores server provided separator.

Cite? Because no, that is not my experience at all.

From my maildir on my FreeBSD server:

# ls -lnd .root*
drwx------  5 89  89  512 Oct 14 05:16 .root
drwx------  5 89  89  512 Oct 14 02:02 .root.44
drwx------  5 89  89  512 Oct 14 02:02 .root.Cron
drwx------  5 89  89  512 Oct 14 05:16 .root.Hastur
… etc

Works fine in Mail.app on Mac OS and has for at least 16 years.

I just tested it with my Dovecot 2.2.36 setup. I did set up an account with '.' as separator and created some folders.
First mail.app succeeded in creating test root folder but when I tried to create subfolder 'test2' under 'test' Mail.app issued
CREATE "test/test2" folder instead of "test.test2". And then failed to list the created folder completely.

Everything worked if I did set separator to '/'