Sent folder in Messages is spammed with notifications (BB-9351)

There has been a lot of discussion when notification spam hits inbox and maybe even emails. The other side of this is that outbox (folder Sent) is filled with notifications I personally did not send. It looks like someone else has subscribed to some notification which I trigger (like pushing to some repo or replying to an issue) and as a result, I get same notification multiple time into my Sent folder but the recipients just differ.

This makes outbox totally useless because you really need to work hard before you can find any private message you sent to someone.

Example: When I created this issue I got 5 new messages into my Sent folder and all recipients were Bitbucket/Atlassian staff members.