Agreed. This is likely why the reasons behind some of the failed messages in T313672 continue to happen. The code which is used to verify that a message could be used against all targets. However, that may be a little difficult to achieve all at once in a timely manner. The current method pushes everything into a job queue, which means that the checks are performed one at at time instead of all together.

In theory Echo could be used to notify the sender of failed messages. This would make it obvious, and IMO the sender likely wants to know about failed messages. Plus the notifications could be sent one at a time but still be bundleable which would prevent spamming.

Ideally this would be visible on the list page itself, rather than when sending a message, so that we could warn or prevent when someone tries to add the invalid title. The main hiccup is interpreting namespaces from the remote wiki on a central wiki...but theoretically that is doable now with TitleValue and MediaWikiTitleCodec?

In T248861#8471662, @Dreamy_Jazz wrote:

In theory Echo could be used to notify the sender of failed messages. This would make it obvious, and IMO the sender likely wants to know about failed messages. Plus the notifications could be sent one at a time but still be bundleable which would prevent spamming.

This was proposed as part of https://www.mediawiki.org/wiki/Extension:MassMessage/Progress_tracking, but no reason to block on that IMO.