Can't refer due to error 'You can start referring other domains only after you change the mx records of your domain'

Can't refer due to error 'You can start referring other domains only after you change the mx records of your domain'

Hi there,

I'm keen to refer a friend to Zoho Mail but when I try this I get the message, 'You can start referring other domains only after you change the mx records of your domain'. And so I'm not really sure what this means. Can anyone help please?

Thanks in advance!
Michelle