Guild Wars 2

To complete the “Hint Completion” acheivement I need the “Mailbox is Full” message to pop up. My mailbox is showing 10/10 in it, but I am still showing that I need the “Mailbox is Full” message under social to come up.
Do I need to delete a message so a new one can come in? Or will the system tell me the mailbox is full and I need to delete old mail to receive new?

I’m doing this too for my achievement. My mailbox is full, but I didn’t get a hint. I re logged and that didn’t do anything. How do we get the hint to pop?

Edit: Just have to delete a message and the hint pops.