Blocklists

A block rejects messages from matching senders at acceptance time, before any filter runs. That is what separates it from a filter rule, which only sorts a message that has already been accepted.

Adding a pattern

Blocking works by pattern, either a single address such as person@company.com or a whole domain such as *@spam.example. Each block can be disabled individually without deleting it.

The two modes

This is the actual decision, and it has consequences:

Stealth (unknown). The sender gets the same answer as for an unknown address. They do not notice the block and do not learn that your mailbox exists at all.

Reject (550). An honest 550 states clearly that the message was refused.

Which one when: against a persistent sender who should not know they were blocked, stealth helps. Where a legitimate sender should notice that their mail is not arriving so they try another route, reject is right. For spam, stealth is usually the better choice, because a clear rejection confirms to the sender that the address exists.