.IP \[bu]
Confirmation of the existence of an \fIMX\fR record for the domain
-part of the address. This is not required; in fact many domains accept
-mail via an \fIA\fR record for e.g. example.com which is used in lieu
-of an \fIMX\fR record. This behavior can be controlled via the
-\fR\-\-accept\-a\fR flag.
+part of the address. NULLMX (RFC7505) records are not accepted. This
+is not required; in fact many domains accept mail via an \fIA\fR
+record for (say) example.com which is used in lieu of an \fIMX\fR
+record. This behavior can be controlled via the \fR\-\-accept\-a\fR
+flag, but note that \fR\-\-accept\-a\fR is ignored for domains that
+have NULLMX records.
.P
These checks are performed in parallel using the number of available