MassMessage regularly fails with a readonly error code on some of the target wikis, for no obvious reason. For large target sets there can be hundreds of failures.
The failure is only logged on the local wiki and not exposed to the sender, making these errors hard to realize and cumbersome to identify (T139380#4460313 has a workaround for the latter).
(Merged duplicate description:)
The last few days, we've had reports of MassMessage not delivering messages to everyone on the target list.
For T213864, two hours after a 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 out of 11 received the issue).
See comments below for many examples and logs
Possibly related (?) T214712: MassMessage delivery of Tech News failing in wikitext on Hebrew Wikipedia
Another duplicate (?) T180378: simplewiki has a lot of MassMessage failures due to "readonly"