Limitation or Delay with zoho free mail account?

Limitation or Delay with zoho free mail account?

Hello to all
I configured several free account to test the service and propose , if necessary , the mail zoho to my customers .

despite the MX record configuration is correct , the mail does not arrive or arrives too late an hour !

I tested the service with an internal mail server , with gmail and hotmail . emails arrive late or do not arrive !

there is some limitation with free accounts ? mail is downloaded or checked every time ?

thank you