DKIM TXT DNS record mail._domainkey exists but verifier.port25.com says it doesn't
I followed the excellent article: https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-dkim-with-postfix-on-debian-wheezy to set up DKIM on my server. I created the two TXT records exactly as required. My domain is called realassurance.com. However the authentication check using a test email to: email@example.com returns the following in the DKIM section stating that mail.domainkey.realassurance.com doesn't exist (see below), when it does. Having read as many blog articles as I could find about this I have ensured the text is in quotes and I am sure it is not a propagation issue as I used mxtoolbox.com (and other similar online tools) to check for mail.domainkey.realassurance.com which was found as a TXT record ok.
Also I note in the report below, it reports the DNS record as TXT (NXDOMAIN) - whatever that means.
No idea what to do next. Can someone please help?
DKIM check details:
Result: permerror (key "mail._domainkey.realassurance.com" doesn't exist)
mail._domainkey.realassurance.com. TXT (NXDOMAIN)
NOTE: DKIM checking has been performed based on the latest DKIM specs
(RFC 4871 or draft-ietf-dkim-base-10) and verification may fail for
older versions. If you are using Port25's PowerMTA, you need to use
version 3.2r11 or later to get a compatible version of DKIM.