I miss the option to send delayed (waiting) messages in the outbox now.
There is no way to force a message to be sent if you have already specified delayed send.
What you can do is to open the message in the Outbox and change the delayed send time to the current or past time.
When I open a delayed msg in the outbox the actual delay should be removed. I then have the option to to send now or delay again via the send button.
Mark all delayed msgs in the outbox, left click and select send now (new option).