Cannot send mail "Mail cannot be sent as the Email Address does not exist"

Cannot send mail "Mail cannot be sent as the Email Address does not exist"

I have an issue sending emails to a particular email address using zoho mail. Clicking send email results in an error prompt of:  mail cannot be sent as the Email Address does not exist. This is only for particular contacts, and I believe the error occurred spontaneously. 

We have previously sent emails to this contact out of Zoho. Any help would be appreciated.