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5.3.0

Message Not Delivered: unknown issue with the receiving system

The message could not be delivered because of an unknown problem with one or more of the recipients' email systems.

Fixes

Double check that the recipient's email address is correct. Wait a few minutes, then try sending the message again.

If necessary, contact the recipient(s) another way (e.g., phone or text message) to confirm their email address.

5.3.0: Official Definition

5.X.X

[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.

X.3.0

[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.3.0:

The destination system exists and normally accepts mail, but something about the system has caused the generation of this DSN.