The maillog is easy enough to follow, but when you understand what all the delay and delays numbers mean then this may help really understand what is going on!
A standard email entry in postfix looks like:
Jan 10 10:00:00 testmtr postfix/smtp[20123]: 34A1B160852B: to=, relay=mx1.example.lan[1.2.3.4]:25, delay=0.49, delays=0.2/0/0.04/0.25, dsn=2.0.0, status=sent
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