In the section of displaying a zone hosted at your free (to customers) DNS service (i.e. "https://dns.he.net/
" interface), I noted what appears to be an error.
I have some hosts in a forward zone that have MX records with a priority of zero. Instead of printing a zero, the output routine is printing a dash in the priority column. The dash should be for records which have no priority field, but apparently, you're using it for priority zero as well, which is incorrect.
Under BCP, Internet hosts which do not receive mail may (or should) have an MX record with priority zero and pointing at the DNS root. Those sending only mail internally to the host should have an MX record with priority zero pointing at localhost (which should be rejected under SMTP by all other hosts as unroutable or as a routing loop).