First Confluence
When Atlassian released Confluence 7.0, among other libraries, the Java mail library was updated to a newer version. Unfortunately, it turned out that this version had an issue that made it impossible for S/Notify to properly work any more. It took a while until the problem was finally fixed with another update of the Java mail library in Confluence 7.5.

Then Jira
Jira was still using an old version of the Java mail library that needed an update, too. So in Jira 8.10, Atlassian updated it to the version that was known not to have the issue like in Confluence 7.5. Good job!

However (you knew there would be a however), they did not remove the problem version – maybe from an earlier update of the library, then forgotten after the second? We don’t know, but this lead to strange and unpredictable effects. For example, S/Notify sometimes works and sometimes doesn’t, depending on which library has been loaded – there was no foreseeable pattern.

Atlassian fixed that problem relatively fast in Jira 8.12.

Now Bitbucket
Bitbucket was on an old version for quite long, when Atlassian eventually decided to update the Java mail library. Unfortunately, it looks like the lessons learned from Confluence and Jira had been forgotten. That’s probably why, in Bitbucket 8.16, Atlassian decided to update again to the broken version 1.6.2 of the Java mail library!

We’ve instantly filed a bug report, and now we’re hoping – once again – for Atlassian to get this issue fixed very soon.

Actions To Take
In the meantime, either just don’t upgrade to Bitbucket 8.16+, or, if you need to, after the install of Bitbucket, apply our proven fix as explained here.