email error when placing an order...

Hi.

We are setting up a new zencart shop. it is located in the same server as the previous cart, with the same settings as the other site in terms of email settings.

We are using PHP in mail settings of zencart. The customer who places the order gets a confirmation email without any problem, however, the admin gets this error instead:

reason: 551 This mail server requires authentication before sending mail from a locally hosted domain. Please reconfigure your mail client to authenticate before sending mail

why is this so? if PHP mail works fine and sends to customer without issues, why are the emails to the seller failing and getting a 551 error instead?

If I try to log into console of the serer and send mail to the admin email, it sends without any issue.

any suggestions?