I do a lot of my RP in private messages and I just had the idea what if we could add tags to private messages, being able to sort out my inbox as "RPs" and "OOC chatter." Would make things feel a lot less cluttered, but I don't know if it's possible at all.
Yeah, I use a 2-PM system with "<Story name here>" for the actual RP and "<Story name here> OOC" for OOC chatter. If you're the one who sent the first message in a PM thread, you can re-title it using the Edit button.
Underneath/next to the "Conversations" header, next to "Start conversation", there's the "Conversation folders" button you can use to add folders which basically function as tags. From your inbox you can select and move your messages to a specified folder, and at the top of each individual message thread there's a drop-down menu with folders.
In a similar manner you can star PM threads to e.g. indicate which conversations are still active.
Like Siph said, you should be able to create conversation folders and then add the folder tag to your PMs for sorting. I have them for OOC, Active RPs, Inactive RPs, and Staff stuff. I would never find my RPs otherwise.
I will add usually when writing in PMs I have myself or my partner (whoever starts) put the words [RP] like that in the title which makes it easier to sort through them as well.