[openstreetmap/openstreetmap-website] Replying to emails does not deliver message to OSM user (Issue #4460)

[Milan Cvetkovic]

### URL

_No response_

### How to reproduce the issue?

1. OSM user `usera` leaves a private message to OSM user `userb`.
2. `userb` receives an email message from OSM with the content of the message that `usera` sent
3. `userb` replies using email client to the message, email client send the email to `usera <m-dddd-cccc at messages.openstreetmap.org>`. The email is successfully sent, as reported by MTA.
4. `usera` never receives the message. `userb` never sees their message in the outbox on OSM web site.

### Screenshot(s) or anything else?

I have unsuccessfully tried to reproduce the issue locally, by hand-crafting the email content and piping it to `script/deliver-message`. 

Here are the headers of one reply message as sent by the email client (some portions are replaced with word "redacted")

```
Date: Fri, 05 Jan 2024 12:51:42 +0100
From: <redacted>
To: redacted <m-1155857-VpmyK4S3 at messages.openstreetmap.org>
Subject: Re: [OpenStreetMap] <redacted>
User-Agent: K-9 Mail for Android
In-Reply-To: <redacted>
References: <6597e31f6a8d9_20882e1a902757c4 at spike-01.openstreetmap.org.mail> <redacted>
Message-ID: <001551C3-090C-4D7F-ABE0-D489222AD88E at redacted>
MIME-Version: 1.0
Content-Type: text/plain;
 charset=utf-8
Content-Transfer-Encoding: quoted-printable

body
```

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https://github.com/openstreetmap/openstreetmap-website/issues/4460
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