The mail log looks ok apart from this:
Dec 28 18:04:29 server postfix/smtp: 9BC329137B7: to=<firstname.lastname@example.org>, relay=uswgco33.uswest.com[22.214.171.124], delay=6, status=bounced (host uswgco33.uswest.com[126.96.36.199] said: 550 5.7.1 <email@example.com>... Rejected: 188.8.131.52 listed at http://www.mail-abuse.com/enduserinfo.html (in reply to RCPT TO command))
which means your IP is on a black list. Is it a dynamic IP address? Many email servers block emails that originate from dynamic IP addresses.