Timo confirmed this type of message (see below) breaks iOS mail clients regardless of the IMAP server used. So it is almost certainly a client issue.

We are in contact with the Apple Mail developers and have provided this debug information to them.

michael

On 02/10/2025 7:19 AM MST Timo Sirainen via dovecot <dovecot@dovecot.org> wrote:

On 10. Feb 2025, at 15.24, Florian Effenberger via dovecot <dovecot@dovecot.org> wrote:

...

Hello,

frido--- via dovecot wrote on 10.02.25 at 14:07:

...

The problem as described at the start of this thread is not solved in iOS 18.3. A bit more testing shows that it only appears to happen with attachments around 1MB and bigger.

there seem to be two kind of problems, at least on my devices.

I'm also wondering if there are multiple issues. I have one way of easily reproducing, but it has nothing to do with IMAP IDLE. Here's a test email which causes the hang - snipped it a bit in the middle which you can fill out yourself with:

dd if=/dev/urandom bs=1024 count=3432 | base64 -w76

If you want to test multiple times, change Message-Id for every delivery. This mail can be simply saved to INBOX, e.g. with doveadm save:

To: <user@example.com> Subject: testing Content-Type: multipart/mixed; boundary="foo" Date: Tue, 17 Dec 2024 06:46:17 +0000 From: <user@example.com> Message-Id: <fooab@example.com>

--foo Content-Type: multipart/alternative; boundary="bar"

--bar Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8

asdf --bar--

--foo Content-Description: test.pdf Content-Disposition: attachment; filename="test.pdf" Content-Transfer-Encoding: base64 Content-Type: application/octet-stream; name="test.pdf"

lBjZO0luFuyUiOqQ8KPTTpMyPoNhlSTPPJauDuCVSwbZgaNyhnObul5qrdyPKo0NGiQKA3Kha34A <61642 lines of random> 4cNyk39T2wMsnQavfl8LXwG1vNQylpgmSN4p6fOpmGqQ

--foo--

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