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Did you have this domain name inside your domain at one time?
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No this is to an external company. Never had this before I started hosting my own email, and obviously their server is exchange based, but not sure. Used mxtools and we are on 2 possible blacklists, but besides that all looks well.
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I am using postfix on this end and can send to anyone except one of our recipient domains. I set up a separate domain.com dns zone in the DNS server to point to the local IP address instead of the external because of mobile workers coming in. Is that going to affect anything if I just use the IP address for my internal users pc's that do not leave the building?
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Wondering why I am having this issue with a certain domain sending email.
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Wondering why I am having this issue with a certain domain sending email.

Did you have this domain name inside your domain at one time?

No this is to an external company. Never had this before I started hosting my own email, and obviously their server is exchange based, but not sure. Used mxtools and we are on 2 possible blacklists, but besides that all looks well.

I am using postfix on this end and can send to anyone except one of our recipient domains. I set up a separate domain.com dns zone in the DNS server to point to the local IP address instead of the external because of mobile workers coming in. Is that going to affect anything if I just use the IP address for my internal users pc&#39;s that do not leave the building?

receiving dns; mail.messaging.microsoft.com Diagnostic-Code: smtp; 550 Requested action not taken: mailbox unavailable

An email client, email reader or more formally mail user agent (MUA) is a computer program used to access and manage a user's email. A web application that provides message management, composition, and reception functions is sometimes also considered an email client, but more commonly referred to as webmail.

Email Software

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

The Domain Name System (DNS) is a hierarchical, globally distributed system responsible for associating the name of a computer, service or other resource into an IP address for connecting to the Internet or a private network. Most prominently, it translates domain names to the numerical IP addresses needed for the purpose of computer services and devices worldwide.