A handler can be invoked at the hooking points described below.
Before queuing
Handlers can be called before the message is placed in the MTA queue. If
this is a message being received by SMTP, the handlers are called
synchronously during the SMTP session.
At this point, the message can have multiple recipients listed in the
envelope. The handler can only take actions upon the message as whole,
i.e. it cannot, for example, reject the message for some recipients
while accepting it for the others.
Before local delivery
These handlers are called after reading a message from the MTA queue and
deciding to deliver it to a local recipient. Note that messages to
nonexistent local addresses are handled, too.
At this point, the message has exactly one recipient. Messages directed
to multiple local recipients are split at an earlier stage. The handler
can take actions upon the message directed to one recipient without
affecting sibling messages directed to other local recipients.
Before remote delivery
These handlers are called after reading a message from the MTA queue and
deciding to deliver it to a remote host.
At this point, the message can have multiple recipients. This happens
only when the message is to be delivered to the same host for all of
these recipients. If the message has multiple recipients for which
remote delivery to different hosts is needed, such a message is split at
an earlier stage.
Before SMTP data
These handlers are called after the envelop is sent, but before sending
the body of a message (before the DATA command of SMTP protocol).
Used by services that process only the messages headers, not their
content.