Out of order no, failing and having to manually re-send which makes it out of order is acceptable.

I find that unacceptable and frustrating, personally. If a message fails to send, I want the client to hold back any later messages until the failed message is resolved somehow. It should auto-retry and (hopefully) eventually succeed, or I can manually delete it and "release" the following messages.

I do so with the expectation that they should arrive in order if no one fails, but apparently it is "debatable" if this is a reasonable expectation