The last few days, we've had reports of MassMessage not delivering messages to everyone on the target list.
For T213864, two hours after the message was sent out (14:47 UTC, 16 January 2019) to https://meta.wikimedia.org/w/index.php?oldid=18788945 a manual check shows that some of the targets have received the message, but multiple wikis have not. The queue is said to be empty.
At https://meta.wikimedia.org/wiki/Talk:Tech/News/2019/03 @IKhitron has reported messages not being delivered (seen on he.wp, where 1 user over 11 received the issue).
So far this means that some communities do not get the message that they can't edit the wikis for a short period of time tomorrow, and that others do not get the updates of technical changes, so it's a somewhat urgent problem.