[Exim] meaning of -> , => , <= , ==, ...

Hello,
I am not a real MTA Admin, i'm still learning ...
I would like to know the meaning of those signs i find when tailing the
exim logfile.
Is there a simple way to interpret them ?
What tool should be used for that ?
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Rakotomandimby Mihamina Andrianifaharana
Tel : +33 2 38 76 43 65
http://www.rktmb.org/Members/mihamina