That's Proton Mail blocking your message from being sent. Have you used this email for anything else before? They really don't like it when you only send outgoing messages. Especially if you are a free account.
Yeah well, I 've got around 50 emails outgoing and around 30 incoming. I am not sure what the problem is, but I guess you 're right.
Anyway, I think I may try with another email as well, but the only one which I think I could use has even less emails.