Mails don’t always send

What happened?
I pressed send on a mail, but it did not send (didn’t appear in send and in the mail provider app)

What did you expect to happen?
Send the mail

Steps to reproduce
Compose, send, and look in the send folder

What mail provider(s) are you using?
Outlook, but maybe also a problem with iCloud

What version of iOS?

What build number of Big Mail?