This has now been resolved, please see below from JiscMail:
"We are pleased to inform you that, as of 14:00hrs on Wednesday, 11 January 2023, we believe the root cause of the issue (the blacklisting) to have been resolved, and are in the process of resubscribing all affected users. We expect this to be completed by no later than 17:00hrs on Wednesday, 11 January 2023.
Despite these issues having been addressed, subscribers and list owners may continue to recieve error messages that have been "queued" by mail servers outside of our direct control. These are expected to have fully cleared by Friday, 13 January 2023.
We apologise for any inconvenience caused during this time."
