I know your pain. My inbox is 235 pages, my outbox 186 pages. Have you considered downloading your PMs and managing them locally? See
Backup/Sort Private Messages/PM/DM (I haven't tried this).
Oh wow, I didn't realize this thread was so old until I merited OP--but whatever, he's got a good point even a couple of years later. The messaging system is one of only a few things about bitcointalk that I'd like to see updated.
And LoyceV, thanks for that suggestion. I've got a few PMs that I'd like to save, but the rest have outlived their usefulness and I guess I missed that thread you linked to. Oops.
Of course, considering we won't get a new forum software anytime soon, it's a very good idea.

C'mon, what makes you say that? It's ready to be released any day now. Any day.
What a quite relief to hear this from you sir,that a new forum software is ready and will soon be released in no time. I only have few PM because I am new here but I think that from my observations it will be nice if the inbox and outbox messages are separated just as suggested by my seniors and forum coordinators.