(invalid/non-working email address)

When sending emails to our club two members get a message similar to the following:

Emails to the the following members were skipped
for the reasons listed below:

Gail Summers (invalid/non-working email address)

The two members are Gail and Ken Summers (me) (gail@gksummers.net and ken@gksummers.net)
What can we do to get them both back in the system's good graces?

Ken

Gail and Ken host their email in a datacenter that frequently spams FTH and the IP was blocked.
I have removed the blocked for now it may happen again.