When I want to send a message from my Zoho mail to another mail, I keep having a message said "The email address is invalid". What should I do?

When I want to send a message from my Zoho mail to another mail, I keep having a message said "The email address is invalid". What should I do?

This is really bothering me...

I tried sending a message to my admin email and that worked but whenever it's a new email it won't send it and the popup says  "The email address is invalid".