My mail server is pretty rigorous and will fail to accept email under certain conditions.
Nov 4 15:46:35 www postfix/smtpd[2692876]: NOQUEUE: reject: RCPT from vmx.spamcop.net[184.94.240.112]: 450 4.7.1 <prod-sc-www02.spamcop.net>: Helo command rejected: Host not found; from=<service@admin.spamcop.net> to=<me@mydomain.com> proto=SMTP helo=<prod-sc-www02.spamcop.net>
prod-sc-www02.spamcop.net does not resolve to an IP, therefore your mail is dropped. Please fix this. Thank you.