There doesn’t seem to be a clear question or answer on this Discourse so since google’s asking me for logs for STMP and POP for reporting to their Report a delivery problem between your domain and Gmail - Gmail Help form.
If it logs, check /var/logs/mail, etc.
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You’re a star @murgero its people like you who make not only this world a better place but forums actually useful. I ended up finding that command buried somewhere deep in someone else’s thread but now this’ll be there for someone else if they end up wanting to know the same thing. 
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haha no worries:
tail -n20 /var/log/mail to get last 20 lines (change 20 to any number to get that many lines)
A little more help for you. 
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I’ll take that advice and up you one more with a tail -n20 /var/log/mail |gist-paste -p to make it instantly shareable.
Never used it before but hell ya!