If Firefox Private Network Browser-level Protection is enabled in outlook.com email sessions -- the outgoing email will not send. "Sometimes" the email will go through but takes forever to send. Most of the time it just says, "Still sending..." at the bottom. After a while outlook.com times out saying, "we couldn't send your email" at the bottom. It'll then say, at the top, "This message can't be sent right now. Please try again later." If you disable Firefox Private Network Browser-level Protection -- click "Send" -- voila -- the email sends instantly. Doesn't matter where the recipient domain is -- this can even help when sending test messages to myself.