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This FAQ item explains how the White and Black listing section works.

The White and Black listing section allows blacklisting or whitelisting email addresses based on the Envelope Sender/Recipient.

Generally, the Envelope Sender/Recipient are the addresses that are used in the communication between email servers. The Envelope Sender would show the sender that is set by the sending email server. On our servers, the Envelope Sender is set as Return-Path of the message, and when viewing the source of an email message received on our servers, the Return-Path header would be the Envelope Sender from the SMTP communication between email servers. The Envelope Recipient would be the mailbox that will receive the message. Note that you may even not see this Envelope Recipient in the message itself. For example, if the sender uses a blind carbon-copy (BCC), it is possible that you would see a TO header in the message source, while the message could be received in a completely different mailbox (due to the BCC used when sending).

On the other side, the From and To addresses in the message headers are set by the sender, and they could be easily forged. That's why most spam messages have incorrect From and To headers. Generally, message headers are not a reliable way of blocking spammers, because manipulating message headers is a very common spam-sending technique.

The White and Black listing section works on two levels:

  • Global level for all mailboxes on the account, through the Edit Global Mail Settings of the hosting Control Panel's Mail Manager section.
  • Local level for each mailbox, through the Filters section next to the mailbox.

If you want to block messages based on From/To headers, you can do this through the Header Filters section.