THIS IS A WARNING MESSAGE ONLY. YOU DO NOT NEED TO RESEND YOUR MESSAGE.

An automated message that notifies you about your message to some e-mail is delayed or fails. It used to appear on Gmail in mid-2010s, now it appears on Protonmail.

I don't know the purpose of using uppercase letters, like that's unprofessional.

THIS IS A WARNING MESSAGE ONLY. YOU DO NOT NEED TO RESEND YOUR MESSAGE.
Delivery to the following recipient has been delayed: (email address)
Message will be retired for 2 more day(s)