For many reasons, it is not recommended to simply delete all erroneous email addresses that have been acknowledged with a 5.x x SMTP status code from the central data. Rather, the text in the "Reason" field should also be evaluated. This allows you to build up a certain classification at the same time.
To detect "Unknown User", we recommend searching for the following keywords / strings in the SMTP reason text:
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Disabled
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No such user
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No such address
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No such recipient
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No such mailbox
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Mailbox unavailable
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User doesn’t have
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User not found
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User unknown
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Unknown user
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Unknown or illegal alias
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Invalid recipient
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Invalid user
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Invalid mailbox
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Does not exist
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Recipient unknown
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Recipient not found
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Bad destination
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Unroutable address
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Address rejected (attention, because often used in case of policy errors!)
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Recipient rejected (attention, because often used in case of policy errors!)
To detect syntactical or DNS errors (no A or MX record etc.), we recommend searching for the following keywords / strings in the SMTP reason text:
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Host or domain name not found
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Syntactic incorrect address
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Network is unreachable
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Connection timed out
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Connection refused
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No information available
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Loops back to myself
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No route to host
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Malformed or unexpected name server reply
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Timed out while receiving the initial server greeting
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nullMX
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[empty reason text]
For users with a permanently full mailbox, we recommend searching for the following keywords / strings in the SMTP reason text.
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quota
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mailbox full