Goutham

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You can do this with GSuite. &nbsp;<br />
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<a href="https://support.google.com/a/answer/9175444?hl=en&ref_topic=2683824" rel="ugc nofollow">https://support.google.com/a/answer/9175444?hl=en&ref_topic=2683824</a><br />
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I have looked in my own admin console to verify. What I found was <a href="https://admin.google.com/AdminHome?hl=en#ServiceSettings/service=email&subtab=filters" rel="ugc">https://admin.google.com/AdminHome?hl=en#ServiceSettings/service=email&subtab=filters</a>&nbsp;and see the option for <b>Restrict Delivery</b> - <i>Restrict the domains that your users are allowed to exchange email with.</i>
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You can do this with GSuite.  

https://support.google.com/a/answer/9175444?hl=en&ref_topic=2683824 [https://support.google.com/a/answer/9175444?hl=en&ref_topic=2683824]

I have looked in my own admin console to verify. What I found was https://admin.google.com/AdminHome?hl=en#ServiceSettings/service=email&subtab=filters [https://admin.google.com/AdminHome?hl=en#ServiceSettings/service=email&subtab=filters] and see the option for Restrict Delivery - Restrict the domains that your users are allowed to exchange email with.

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Dear Experts<br />
We are using g-suite/google aps for the email services. We have following requirement that is to restrict the user accounts such that they should be able to send the emails to all the users within the same domain and should be able to send to external domains users of 4 domains and restrict for all other domains.<br />
Please suggest is possible if yes then what should be done.<br />
If above is not possible with google aps/g-suite then we have to setup email server on-premises and if we go for the on-premises zimbra email server can we achieve this please suggest, thanks in advance.
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Dear Experts
We are using g-suite/google aps for the email services. We have following requirement that is to restrict the user accounts such that they should be able to send the emails to all the users within the same domain and should be able to send to external domains users of 4 domains and restrict for all other domains.
Please suggest is possible if yes then what should be done.
If above is not possible with google aps/g-suite then we have to setup email server on-premises and if we go for the on-premises zimbra email server can we achieve this please suggest, thanks in advance.

email accounts of Google aps apply restrictions to send only for few domains and block rest

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

Google Workspace is a suite of cloud computing productivity and collaboration software tools and software. It includes Google’s popular web applications including Gmail, Drive, Hangouts, Calendar, and Docs. While these products are available to consumers free of charge, Google Workspace adds business-specific features such as custom email addresses at your domain and storage for documents and email.