4961: Outlook threading breaks for ReviewBoard emails


What version are you running?

What's the URL of the page containing the problem?

It is not a public URL

What steps will reproduce the problem?

What is the expected output? What do you see instead?

What operating system are you using? What browser?


Please provide any additional information below.

Some ReviewBoard notification email threads break out into two or three separate threads. This happens seemingly at random, and only for emails from ReviewBoard. Issue occurs across Outlook web as well as desktop, and for all recipients of that ReviewBoard request (as long as they have Outlook threading enabled).

We suspect the issue is: Threading in Outlook depends on UUID in the Thread-Index header, then they are grouped no matter whether the subject is the same or not, but this has to follow the format nevertheless.

Reviewboard is breaking that value and that's why it fails to thread in Outlook across platforms.