No invite being sent to 1 user, and not showing in zoho desk

No invite being sent to 1 user, and not showing in zoho desk

Hi,

I have sent out many many new user invites from Zoho desk, but this one user (email address) is not
a) being received (not even in junk) and
b) does not show as invited on zoho desk

I read there may be a way to send a manual URL can you help please?

Regards
Ian