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Hello,<br />
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The links below might be helpful for you mate<br />
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<a href="http://exchangeserverpro.com/mx-record/" rel="ugc">http://exchangeserverpro.com/mx-record/</a><br />
<br />
<a href="https://forums.cpanel.net/threads/why-didnt-my-email-get-delivered-or-where-did-my-email-go.387312/" rel="ugc">https://forums.cpanel.net/threads/why-didnt-my-email-get-delivered-or-where-did-my-email-go.387312/</a>
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I would like to know if the secondary mx record will received the email which are unknown on my primary mx record or if the sender will received a 550 error by the primary email server.
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I would like to know if the secondary mx record will received the email which are unknown on my primary mx record or if the sender will received a 550 error by the primary email server.

Forward unknown email receipient to secondary mx

Interactions between email servers and clients are governed by email protocols. The three most common email protocols are POP, IMAP and MAPI. Most email software operates under one of these (and many products support more than one).  The correct protocol must be selected, and correctly configured, if you want your email account to work.

Email Protocols

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.

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.