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Inbound mail processing
When an SMTP connection is established, the connecting server is checked against a database of servers from which mail is not accepted. If found there, the connection is rejected permanently with a 553 error that includes instructions and a link to request removal of the server from the database. Details
If not rejected, the connecting server is checked against several Real-time Block Lists (RBLs) and rejected with a 553 error if found. Details
Next the incoming mail is scanned for viruses and spam content. If either is found, the connection is rejected permanently with a 554 error. Details
Remaining mail is processed for delivery.
If the mail is addressed to an invalid recipient, the original message is bounced to the sender and also saved to a "spam trap" folder for further processing. If the bounce is rejected, the resulting double bounce is discarded.
The "spam trap" folder is analyzed periodically and the delivering IP address of the messages to invalid senders is evaluated. If the delivering server is in a list of "acceptable" countries, the message is reported to the abuse address listed in the Regional Internet Registry (RIR) for that IP address. Details
If the message is not from an "acceptable" country, the entire netblock as listed in the RIR is added to the rejection database used at the initial connection. Details