I sent a message to someone on the forums, but it isn't showing in my inbox like it normally does.

[RatB0Y68]

If I remember correctly, you used to be able to see your sent messages after you sent them.

Now I am wondering if I even sent it at all, I'm sure I did. Will it show up in my inbox when they answer?

[Pickysaurus]

If you did send it successfully it would appear in your inbox. 

The only time the message is removed is if both parties delete it.