.mobi domain email invite issue

I've tried to send an invite to a user with a .mobi domain and I receive an "Undelivered Mail Returned to Sender"... the email address is being cut short to .mob

This is the mail system at host postmaster.twitter.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<user@example.mob>: Host or domain name not found. Name service error
for name=example.mob type=A: Host not found</postmaster>