Open Radar

 
Summary:
Every now and then, Mail will mix mails from different threads into a single thread. Interestingly, it will not just merge two complete threads, but only sort some of the mails from thread A into thread B.
In essence, this will make it impossible to be sure to catch all mails when following a mail thread, since single mails from a conversation may incorrectly be moved to a different, completely unrelated conversation.

This pretty much renders the threaded mail view unusable for larger mail conversations, because it became so unreliable that one will have to run an additional search to make sure all the messages from a conversation.


I tried to find a pattern, but so far, I have not been successful in identifying anything that might cause this behavior. The mails that were incorrectly sorted into another mail thread had
	- completely different subject line
	- different set of addressees, sometimes even completely distinct addressees
	- completely different message IDs
	- were not started by responding to mails from the other thread, but were started as responses to messages from their own thread
	- distinct „In-Reply-To“ headers
	- no matching message IDs in the „References“ header


Steps to Reproduce:
Can’t provide steps to reproduce, but it happens frequently enough to be a real issue.

Expected Results:
Unrelated mails should be sorted into distinct threads.

Actual Results:
Some unrelated mails appear in a mail thread, whereas other mails from the unrelated thread continue to appear in their own thread.

Regression:
Problem started to appear after I updated to Yosemite. 
My colleague has the same issue since updating to Yosemite - seems to be an issue with Mail on Yosemite.