Hello, I am receiving the following error each time I try to send mail from my zoho account. There are only 5 users on the free mail plan account and none of them are exceeding the 5Gb limit.

Here is the error code. Kindly guide me as to where the problem exists.

Error type: "Unable to send message;Reason:554 5.1.1 Mail sending blocked for the domain(s):[gmail.com]”

TIA!