Oh, there has been an update: they changed their mail address. Use support@mullvadvpn.net now.

> Thanks for contacting Mullvad VPN support.
>
> Please be aware that since February 2024 we host our own secure email server with a new address.
https://mullvad.net/en/blog/we-now-self-host-our-support-email
>
> support@mullvadvpn.net
>
> Please re-send your request to this email (using English or Swedish) and we'll be happy to assist you there.