Message Not Delivered: Email systems were incompatible
The message could not be delivered because the sender's email program and the recipient's email program are using incompatible versions of software.
Fixes
Try sending the message again in a while, since this problem might be temporary, or already in the process of being fixed.
If this error persists, contact the email provider for help.
5.5.5: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.5.5:
A protocol version mis-match existed which could not be automatically resolved by the communicating parties.