Create a rule

A rule created here is ON, and starts acting on the next message. That is the right default for a call somebody made deliberately, and it is the opposite of the MCP createRule tool, which writes the same rule DISABLED because a model that decides to archive mail should not have it archiving before a person has read the rule back.

A duplicate name on the same connection is rule_name_taken, a 409. Names are how a rule is recognised in a run log and in the settings screen, so two rules called "Newsletters" is a report nobody can read.

The 101st rule is rule_limit_reached, a 422. The cap is a guard against a script in a loop rather than an accounting boundary, and it is not locked. Two creates racing at 99 can both succeed.

A matches pattern has to carry at least two alphanumeric characters of its own, the same bar the blocklist applies. A bare * is refused at write time rather than accepted and then quietly matching every message that will ever arrive, which is an outage rather than a rule.

reject is refused at write time unless the same rule carries at least one envelope_from condition: reject_needs_envelope, a 422. A 550 answers whoever handed us the message, and on a mailing list that is the LIST, which reads the refusal as a bouncing subscriber and unsubscribes the reader from something they only wanted one person to stop posting to. Even with the condition written, a match that came only from the header identities downgrades to filing under Spam, because the envelope is the only identity a refusal can honestly be aimed at.

A rule-driven forward goes out through the send path, which REBUILDS the message: the original DKIM signature does not survive, and neither do exotic parts, unusual headers or anything past the outbound size ceiling, which a 25 MB message with attachments will exceed. It is a copy of what arrived rather than the message that arrived. The address is checked when the rule is written, so an unverified destination is a 422 on the call rather than a rule that silently drops every tenth message.

reply will not answer a machine. It is suppressed when the message carries Auto-Submitted (other than no), Precedence: bulk|list|junk, List-Id, List-Unsubscribe, X-Autoreply or X-Autorespond, when the envelope sender is empty (the shape every bounce takes) and when the headers could not be read at all. On top of that, one sender gets at most one auto-reply per 24 hours from a given mailbox. Two mailboxes with reply rules and no guard mail each other until somebody notices.