Delayed Message: Email systems were incompatible
The email message has not yet been delivered because the sending email program and the recipient's email program are using incompatible versions of software.
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.
4.5.5: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
A protocol version mis-match existed which could not be automatically resolved by the communicating parties.