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Email domains? &nbsp;All of them.<br />
<br />
Windows Domains? As far as I know, none of them.
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Email domains?  All of them.

Windows Domains? As far as I know, none of them.

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What version of Microsoft Exchange Server allows <br />
me to host multiple domains with the same USERNAME ?<br />
<br />
Example<br />
1. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;One Server<br />
2. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5 domains<br />
3. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;John Doe works for company #1 and has jdoe@company1.com<br />
4. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Jane Doe works for company #2 and has jdoe@company2.com
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What version of Microsoft Exchange Server allows 
me to host multiple domains with the same USERNAME ?

Example
1.	One Server
2.	5 domains
3.	John Doe works for company #1 and has jdoe@company1.com
4.	Jane Doe works for company #2 and has jdoe@company2.com

Microsoft Exchange Server -- host multiple domains, same USERNAME ?

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

Microsoft 365 Enterprise E5 is the most feature-rich version of Microsoft 365 and consists of Office 365, Windows 10, and Enterprise Mobility + Security (EMS). It provides the collaboration, productivity, automation, and security features an enterprise customer may need to support its digital business transformation initiatives in a single stock keeping unit (SKU),
which streamlines its deployment and utilization. For companies that do not need the full feature set, Microsoft offers different versions, such as the E3 SKU at a lower price point and F1 for firstline workers.

Microsoft 365 Enterprise

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.