Duck.com - does anyone have a member using this domain?

I have a club member whose email uses the domain: duck.com. Messages sent to our Members distribution list are not getting through to her. Does anyone have a member using the duck.com domain and how did you solve to ensure they are getting email from the FTH Server?

This is the Undeliverable message that I get back: (I changed the user's handle for her privacy). 

This email address is being protected from spambots. You need JavaScript enabled to view it. appears to be an invalid or non-working recipient email address. (No advertised SMTP servers are accepting mail.)

 *** Please check the email address to make sure it is not misspelled. ***

If you believe this warning is incorrect, please visit  support.toastmastersclubs.org  and post a support request message.
. 

We ALWAYS require your club number to assist you.

They were sent and received 2 emails today.

Sep 18 05:05:57 toastmastersclubs sendmail[28824]: 58I55EoF028266: to=<srehrig@duck.com>, ctladdr= (501/502), delay=00:00:03, xdelay=00:00:02, mailer=esmtp, pri=162300, relay=smtp-inbound1.duck.com. [52.146.152.212], dsn=2.0.0, stat=Sent (Message Queued (B0D3CF4C-3642-4D2A-B22A-B82229393DA1.1))

Sep 18 23:33:06 toastmastersclubs sendmail[3819]: 58INX3VQ003816: to=<srehrig@duck.com>, ctladdr= (501/502), delay=00:00:03, xdelay=00:00:03, mailer=esmtp, pri=120833, relay=smtp-inbound1.duck.com. [52.146.152.212], dsn=2.0.0, stat=Sent (Message Queued (69A3E110-14EC-42C3-9ECB-FA74A452ED47.1))