We send notifications instantly only when there are a small amount. Once there are a large number, it will send them as a background task. Please feel free to suggest this if you want to see them always pushed to a background task, however it has to be noted that all you would be doing there is moving the issue.
I understand it would then mean at least the post goes through, so it could mean you end up with a backlog of tasks, and things are not running behind those items, leading to other issues on the software. So the answer to the question really isnt moving the problem to run elsewhere. Its resolving the problem itself, which in this case is on the email server side of things