Hi Reino. Check in Email and Conferences > Settings > Mail and Conference Settings. If you are set up as the Postmaster, you will not be able to send external mail. If you are not, then there must be some other reason.

By the way, in that same setting is a box you can tick, called "Lock and Send emails Automatically". This means that when you tick Sent in a mail record, then on saving it, it will automatically lock and send it to the queue.