As this is currently intended, if you would like to see this change, I would advise posting in our Feedback section.
However, please keep in mind that it is abnormal what is happening here with your SMTP server. Injection of mail shouldn't cause a delay in the application sending it, even if it's done via background tasks, this could create issues. Especially, if you're sending a large batch of email notifications. Even in a normal case of say 15 email notifications, that normally isn't a problem for the background scheduler to get through in a single run. However, you would hit a timeout on a default PHP configuration if each email takes say 2+ seconds. Therefore, you may need 2 or more runs to get through that, which would then delay other tasks as well.