+Confirmation of the existence of an \fIMX\fR record for the domain
+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
+threads. To increase the number of threads, you can pass the
+appropriate flag to the GHC runtime.
+
+.P
+This will set the number of threads to 25:
+
+.nf
+.B $ email-validator +RTS -N25 < addresses.csv