Bounced email addresses to delete

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:

Disabled
No such user
No such address
No such recipient
No such mailbox
Mailbox unavailable
User doesn’t have
User not found
User unknown
Unknown user
Unknown or illegal alias
Invalid recipient

Invalid user
Invalid mailbox
Does not exist
Recipient unknown
Recipient not found
Bad destination
Unroutable address
Address rejected (attention, because often used in case of policy errors!)
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:

Host or domain name not found
Syntactic incorrect address
Network is unreachable
Connection timed out
Connection refused
No information available
Loops back to myself

No route to host
Malformed or unexpected name server reply
Timed out while receiving the initial server greeting
nullMX
[empty reason text]

For users with a permanently full mailbox, we recommend searching for the following keywords / strings in the SMTP reason text.

quota
mailbox full