What causes the email error: 5.4.6 Routing loop detected

Question

What causes the email error: 5.4.6 Routing loop detected to be returned by an email autoresponder.

Answer

The most common cause of this error is when an email address with an active autoresponder sends an email to another address that also has an autoresponder. This could result in creating a loop if there is not a way to break it.  This error is most likely caused by this value in the email header:

X-Loop

The purpose of this value is to stop an autoresponder loop. If this email error should happen, it is most likely the sender of the message provided this header so that forwarded messages can be rejected and a never-ending email loop can be avoided.