Subscriptions to e-mail accounts at Apple domains suspended

On Jan 5, 2018 the subscription e-mails sent by the Mountain Utilities website to all accounts at icloud.com, mac.com and me.com suddenly started bouncing, being "rejected due to local policy".
So apparently Apple changed something in their spam policy.
As far as I know, there is nothing I can do to make these domains accept these e-mails.
So I have suspended all these subscribers' subscriptions. (More precisely, I have marked their e-mail accounts as no longer functional.)
If any of these people want to receive subscription e-mails again, they should switch to an e-mail address at a different domain.
Sorry for the inconvenience.