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.
Also, be certain to check account status as well and look out for security notifications or warnings within the ProtonMail interface, because accounts may be placed on a temporary restriction basis when logging in repeatedly at different times fails to succeed. In other cases, some email attachments or embedded links may be blocked by one of the many security protocols in place within the ProtonMail system. You may correct some of your email's content to see if the situation has, in fact, been resolved. If the issue does not go away, clear the cache on your web browser or use a different device to access the system. As for persistent issues, it is worth talking to the support of ProtonMail since they can narrow down the problems to certain account limitations or give comprehensive corrective measures.
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