We use a custom plugin to do numeric thread stamping, which has been working well in the code system.
However, it was discovered by Microsoft that Microsoft Outlook and OWA prioritize the Thread-Topic header over the Subject header, which results in the oddness we've experienced in MS365 clients in testing.
During our tasks though a custom handler goes through during the pipeline and modifies the Subject field accordingly, adding a thread tag to it. While this is 'fragile' it does work; the only problem is that Thread-Topic doesn't get altered.
Programmatically, how would we alter the Thread-Topic during the handler's processing to prepend the corresponding prefix/tag so that when the message actually goes out the door it is properly sending with the modified Thread-Topic?
Thomas