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Should port 25 be open between Exchange 2003 (single server) and our 2 Exchange 2010 Hub/CAS servers during coexistence?<br />
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Ex2003 is on our 10.x.x.x old network, and Ex2010 servers on 172.x.x.x new network. Both networks have been bridged together, however when I test Mail Flow should port 25 be open bi-directional on the firewall between the two environments?
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Should port 25 be open between Exchange 2003 (single server) and our 2 Exchange 2010 Hub/CAS servers during coexistence?

Ex2003 is on our 10.x.x.x old network, and Ex2010 servers on 172.x.x.x new network. Both networks have been bridged together, however when I test Mail Flow should port 25 be open bi-directional on the firewall between the two environments?

TCP Ports for Exchange 2003 - 2010 Coexistence

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

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.