Other user can send to me but the rest is unable

Other user can send to me but the rest is unable


I saw one of my friend are using his email on : g*****@gmail.com and it could be opened inside my inbox succesfully.
But another one,... iwa******@gmail.com trying to send excel file, it's being rejected.

With the MailPolicy Violation Error delivering to the address.
Google was trying to send but it was rejected by the server of the recipient....

Any clue? why is my email having this kind of error. And unable to receive some email from other users?

Regards,

Astri