The mailbox software has been installed, but the mailbox does not receive the letter reason _ outside the star virtual host

Reason 1, the MX record for domain name is not resolved
Here is a new network of Mydns features to explain:
such as Domain test.com
First, you want to parse a mail.test.com a record into your server IP in the a record.
Then, in MX parsing, parse test.com, host name write mail.test.com, NOTE!!!
Is parsing test.com MX record, not parsing mail.test.com MX record, this is the most common user error place.

Reason 2, firewall or port conflict
The mailbox requires 25,110 ports, check to see if this port is open, and there is no conflict with the SMTP port in IIS, SMTP can use 26 ports in IIS.