A rule is a condition and an action. When a message matches, the action happens before the message is forwarded.

What a rule can look at

The sender's address, the subject, the alias the message arrived at, the alias description, and the alias label. Conditions combine, so "sender contains noreply and label is shopping" is one rule rather than two.

What a rule can do

• Replace the subject, or the sender name shown in your inbox.
• Block the message, or quarantine it, which holds it instead of forwarding it.
• Remove attachments.
• Add the sender's address, or their whole domain, to your blocklist.
• Forward the message to another address.
• Disable PGP encryption for that message.

Order matters

Rules run in the order shown, and you can reorder them. A blocking rule near the top stops everything below it from being considered, which is usually what you want and occasionally the reason a later rule seems not to work.

Useful ones

• Tidy up the sender name for a service that sends as a random string of characters.
• Remove attachments from an address that only ever receives automated reports.
• Quarantine rather than block when you are not sure. Blocking silently is how a wanted message goes missing for a month.

Rules and the blocklist

The blocklist is the simple version of the same idea: an address or domain whose mail is always refused. Use the blocklist for "never again from this sender", and rules for anything conditional. See Blocking a sender.