The error specifically was:
Oct 28 01:40:02 timbre postfix/smtp: 7FABBEBBBE: to=<email@example.com
delay=5660, delays=5358/0.01/1.1/300, dsn=4.0.0, status=deferred (host lists.ubuntu.com[18.104.22.168] said: 421 chlorine.canonical.com SMTP incoming data timeout - closing connection. (in reply to end of DATA command))
After poking around Google for an hour or so, I finally found a hint
by none other than the primary author of postfix himself, Wietse Venema on postfix list; I dropped my routers MTU to 1492 promptly, restarted daemon and my postfix started delivering mail without giving me any of the error above.
For a moment, I suspected that one of the router between timbre and chlorine was dropping ICMP packets because as mentioned in here
, some of the broken firewall/router deny/reject ICMP packets (which have request for resizing MTU called "MTU discovery") so eventually, the request to scale down MTU wasn't being acknowledged by chlorine and bam! data timeout eventually reported by my postfix.
My postfix log kept reporting SMTP incoming data timeout. After confirming that it was all good over at canonical's mailing list server, i started poking my ISP. Knowing that ISPs are lazy people, I tried to Google insanely with having limited success on postfix's man page.