Hello. This error:
This is the mail system at Proton Mail.
Your email could not be delivered to one or more recipients.
See below for the delivery error and email.
The most common reasons for undelivered email are wrong recipient address or wrong configuration on the recipient mail server.
Learn more about delivery errors in our knowledge base:
https://proton.me/support/smtp-errorsThis is weird because I just sent them a message through my Gmail, and it has gone through with no error, Maybe there is some problem with ProtonMail. Anyway, I have told them to come to this thread as soon as possible and respond to the issue when they see my email