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-errorsHello,
Sorry for the delay in response, your issue has been resolved and I replied to your PM.
We really do have the known delivery problem with proton mail service. The separate notice will be posted once we will have it fixed.
Regards,
CM