This graph shows .ca domains that are advertising the capability to receive the domain.ca email over IPv6. This is advertised by a mail exchange DNS record (MX) that is resolved to an IPv6 address. The reachability of this IPv6 address is also shown.
shows the number of .ca domains advertising the capability to receive their email over IPv6.
shows the subset of the other line ( ). It shows the number of .ca domains advertising the capability to receive their email over IPv6 and are reachable by IPv6. Since the email can also be delivered by IPv4, the difference between the two lines is not causing undeliverable emails.