Unable to send mail - Protonmail.com
Supposing, after a normal attempt at troubleshooting, ProtonMail is still not permitting you to send mails, there are probably other reasons affecting how emails are sent from this particular account. First of all, see if you are within any of your sending limits, because it is known that ProtonMail does put limits on spam and abusers. If you have crossed these limits, then it may take a while for you to be able to click send for more mails to be sent again.