from the Exim documentation.
5. Log line flags
One line is written to the main log for each message received, and for each successful, unsuccessful, and delayed delivery. These lines can readily be picked out by the distinctive two-character flags that immediately follow the timestamp. The flags are:
<= message arrival
(= message fakereject
=> normal message delivery
-> additional address in same delivery
>> cutthrough message delivery
*> delivery suppressed by -N
** delivery failed; address bounced
== delivery deferred; temporary problem
I use it as a kind of pre-processing. If I don't do this, I can't match if an email in the log was the sender or the recipient. I found this old gist on github, so it was'nt nessecary, to start by zero.