You may see this error when an address is provided to the MTA through a browser. Or, the error may be deferred and returned as part of an error return mail message. In both cases, this error message indicates that the MTA is not able to deliver mail to the specified host. To determine why the mail is not being sent to the specified host, you should follow these troubleshooting procedures:
Verify that the address in question is not misspelled, is not transcribed incorrectly, or does not use the name of a host or domain that no longer exists.
Run the address in question through the imsimta test -rewrite utility. If this utility also returns an “illegal host/domain” error on the address, then MTA has no rules in the imta.cnf file and related files to handle the address. Verify that you have configured MTA correctly, that you answered all configuration questions appropriately, and that you have kept your configuration information up to date.
If imsimta test -rewrite does not encounter an error on the address, then MTA is able to determine how to handle the address, but the network transport will not accept it. You can examine the appropriate log files from the delivery attempt for additional details. Transient network routing or name service errors should not result in returned error messages, though it is possible for badly misconfigured domain name servers to cause these problems.
If you are on the Internet, check that you have properly configured your TCP/IP channel to support MX record lookups. Many domain addresses are not directly accessible on the Internet and require that your mail system correctly resolve MX entries. If you are on the Internet and your TCP/IP is configured to support MX records, you should have configured the MTA to enable MX support; see TCP/IP Connection and DNS Lookup Support 12.4.3 TCP/IP Connection and DNS Lookup Support for more information. If your TCP/IP package is not configured to support MX record lookups, then you will not be able to reach MX-only domains.