553 Relaying disallowed

Hi, I'me getting error 553 "Relaying disallowed" when trying to send mails from differents external accounts to my zoho account.

I've seen in a post that if we have the account created at US DataCenter we have to point MX Records to mx.zoho.com instead of mx.zoho.eu.

How can we know which DataCenter or which MX records should we use?

Thanks and regards,

César.-