Now resolved.
There are likely to be many instances of forum emails (notifications, new user requests) failing to be delivered during the next 24-48 hours. When I am confident that the problem has been resolved I will make another announcement.
TL;DR
The IP address of my email server has been blacklisted by Spamhaus Zen. Hence many emails sent by my server are being rejected by recipient servers.
This is due to my static IP address being changed when I moved house on Tuesday, and me just discovering that the new address is not on the list of IP addresses that are permitted to send email.
Thanks a bundle Plusnet - again!!!
Plusnet will be "investigating the issue" one their customer service department re-opens Friday morning, and will report back at some point. Until I hear back, it is not worth the hassle of requesting a delist from Spamhaus, as I'll only end up back on the blacklist if Plusnet haven't got their finger out and removed me from their "non-MTA" list.
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Latest Cumulus MX V4 release 4.0.1 (build 4023) - 16 May 2024
Latest Cumulus MX V3 release 3.28.6 (build 3283) - 21 March 2024
Legacy Cumulus 1 release 1.9.4 (build 1099) - 28 November 2014
(a patch is available for 1.9.4 build 1099 that extends the date range of drop-down menus to 2030)
Download the Software (Cumulus MX / Cumulus 1 and other related items) from the Wiki
Latest Cumulus MX V4 release 4.0.1 (build 4023) - 16 May 2024
Latest Cumulus MX V3 release 3.28.6 (build 3283) - 21 March 2024
Legacy Cumulus 1 release 1.9.4 (build 1099) - 28 November 2014
(a patch is available for 1.9.4 build 1099 that extends the date range of drop-down menus to 2030)
Download the Software (Cumulus MX / Cumulus 1 and other related items) from the Wiki