Kinds and actions

phil/bounce-parser

Kinds

`BounceKind`	Means
`Hard`	A permanent failure against the address: the mailbox does not exist, or will never accept the message
`Soft`	A failure that may pass: a delay, a full mailbox, a message too long, a refusal by a filter or a policy
`Complaint`	The recipient, or the mailbox provider for them, reported the message as unwanted
`Feedback`	An abuse report that is no complaint: `not-spam`, `auth-failure`
`AutoReply`	An automatic reply of the recipient
`Delivered`	A notification of success: delivered, relayed, expanded
`Unknown`	Nothing the parser or the rules recognise

A recipient of a delivery status notification is:

Soft when its action is delayed;
Delivered when its action is delivered, relayed or expanded, or when it has no action and a 2.x.x status;
otherwise, a failure: Hard for a 5.x.x status, unless the status speaks of the message rather than the mailbox — x.2.2 mailbox full, x.2.3 and x.3.4 message too long, x.7.* refused by policy —, which is Soft; Soft for a 4.x.x status; without a status, Soft for an SMTP code below 500 and Hard otherwise, as the action failed says no attempt follows.

A refusal by policy is soft because it says nothing against the mailbox: the same address takes the next message once the sending server is off a block list. A server that writes 5.7.1 for a mailbox that does not exist is corrected by a rule of scope Always on its diagnostic.

Actions

`BounceAction`	The application
`Ignore`	Does nothing
`CountSoft`	Counts a soft bounce against the address
`CountHard`	Counts a hard bounce against the address
`Suppress`	Stops sending to the address now, whatever the counters say
`Unsubscribe`	Unsubscribes the address from what it was sent
`Review`	Keeps the message for a person

BounceAction::for(BounceKind) gives the parser's action: CountHard, CountSoft, Ignore for automatic replies and successes, Unsubscribe for complaints, Review for feedback and unknown messages. Two exceptions: a complaint no authenticator vouches for goes to Review, and a not-spam report is Ignore. A rule sets its own action.

The library counts nothing: thresholds of consecutive bounces, the reset of a counter by a successful delivery, and the suppression list are the application's.