Once you have all the relevant security settings adjusted to your liking, you can activate the mail service. Depending on whether or how you've already configured user accounts, this may also require you to change settings for individual users.
Begin by following these steps to turn on the mail server:
Open Server Admin, which is located in /Applications/Server.
In the sidebar on the left, select your server.
If no services are listed under the server name, click the disclosure triangle next to the server name to reveal them.
If the service names are dimmed, choose Server Connect, type your username and password if they're not already filled in, and then click Connect. The list of services refreshes, and those currently running appear with a green dot next to them.
In the sidebar, under your server name, select Mail.
Click the Settings button on the toolbar.
Click the General tab. The window should resemble Figure 28.4.
If you haven't already done so, type the server's domain name and hostname (likely to be the same) in the Domain name and Host name fields, respectively.
Optionally, to enable push notifications, click the Add button, choose a Mac OS X Server computer (which can be the current server), fill in administrator credentials as requested, and then click Connect.
To enable the SMTP server, click the Enable SMTP check box and then click the Allow ...