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If you are looking for additional mail protection take a look at Mimecast, or a cheaper option would be via 365 <a href="https://products.office.com/en-us/exchange/exchange-email-security-spam-protection" rel="ugc">Here</a>, or SophosPure Message <a href="https://www.sophos.com/en-us/products/puremessage.aspx" rel="ugc">Here</a>
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If you are looking for additional mail protection take a look at Mimecast, or a cheaper option would be via 365 Here [https://products.office.com/en-us/exchange/exchange-email-security-spam-protection], or SophosPure Message Here [https://www.sophos.com/en-us/products/puremessage.aspx]

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My favorite is either Ironport/ESA device on-prem or MX Logic in the cloud
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My favorite is either Ironport/ESA device on-prem or MX Logic in the cloud

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I'd suggest Exchange online protection.
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@David Favor: relay service for sending email
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@David Favor: relay service for sending email

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Best provider for an Exchange Server 2016 &nbsp;SMART HOST
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Best provider for an Exchange Server 2016  SMART HOST

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.