Sorry to ask, is there no subject included in that your pm? Most at times is very difficult to look for a pm without a subject, and if you couldn't find the pm anymore it may be your have mistakenly deleted it. Because I strongly believe, any messages sent or receive will either be at the inbox or outbox, adding additional features would likely make it smarter, something like personal message search button.
Having a subject doesn't make it any much better. I still have to go through all the message I sent. Is there a way to filter messages by subject? I don't know. If such feature existed, there would be no hassle to find out the necessary message.
Use telegram for fast responses, and PM for sign/verify messages
Not a lot of people use telegram. They prefer forum PM only. However, still that doesn't change anything. Finding a sent message requires quite a long time.