Exchange 2010 can't send us emails?

Question

Mail server

Good afternoon, recently, one of the organizations that is on our service wanted an exchange mail server, they wanted to do it. But unfortunately, the first jambs have recently begun to appear, namely,
mail from the sender does not reach the server, the following comes in response to the sender:

The mail system
<****@***.**>: host mx.****@* **.**[88.99.***.57] said: 554 5.6.0
Invalid message content (in reply to end of DATA command)
****@***.** - our dns name

Answer 1

Here is a small man page on SMTP errors