We now support messaging offline users!
No longer will you need to wait for your friends to come online to talk to them! It's been requested for years, and after a lot of work, I've introduced offline messaging! Simply chat in the PM window like you normally would, and after a quick confirmation the message will be off, to be received by the target user when they log back in. For more information, see this post.
—mia on Nov 9, 2023