Apologies everyone, I only just noticed that for four months incoming emails haven’t been working. This includes emails to the support contact address, as well as any replies made via email.
Everything should be working now. I’m very sorry if anyone was affected by emails not being received.
At some time since March the mail-receiver seems to have stopped running again. (I think exim4 somehow got installed and took priority of port 25.) I have started running it again. Apologies to anyone affected by this.
The last email the forum received was just before I updated the server to Debian 12, so I conclude this was in fact the cause.