The pmg-log-tracker extracts information out of the syslog to match all entries
related to a single mail. It does so by using the PID of 'smtpd' entries in the
log as key for the SEntry structs. The QID, an ID consisting of at least 2 all
upper case hex digits, is used for both QEntry structs and FEntry structs.

As a rule of thumb, SEntrys are used wherever the services 'postscreen' and
'smtpd' are involved. QEntrys on the other hand are only used when a queue is
involved (e.g. after-queue filtering passing the postscreen checks,
before-queue filtering when the mail is 'accepted'). FEntrys are used whenever
the filter (pmg-smtp-filter) is involved.

The typical flow of a mail (after-queue):

1. postscreen (NOQUEUE -> mail finished)
2. smtpd (pid matching)
3. cleanup -> qmgr (mail in queue, QID matching)
4. pmg-smtp-filter (rule system, accept/block, long (Q)ID matching)
5. lmtp/smtp/local/error (QID matching, filter ID matching in lmtp)
6. qmgr ('removed')
7. smtpd ('disconnected from')

Both 6 and 7 are required before the mail can be printed completely.
6 and 7 can be logged in arbitrary order.

The typical flow of a mail (before-queue):

1. postscreen (NOQUEUE -> mail finished)
2. smtpd 1 (pid matching)
3. pmg-smtp-filter (rule system, accept/block, long (Q)ID matching)
  - on accept match the QID
4. smtpd 2 (pid matching)
5. (optional, only on 'accept') cleanup -> qmgr (mail in queue, QID matching)
6. (optional, only on 'accept') smtp (QID matching)
7. (optional, only on 'accept') qmgr ('removed')
8. smtpd 2 ('disconnect from')
9. smtpd 1 (proxy-accept/proxy-reject, filter (Q)ID matching)
10. smtpd 1 ('disconnect from')

7, 8 and 10 are required before the mail can be printed completely.
7, 8 and 10 can be logged in arbitrary order.