In Google Business Apps, admin and owner of company can track, search or can get copy of all incoming and outgoing email of your domain users by different ways. Use of such type of option is primarily meant for auditing, for compliance purpose only. You need to keep your country law and organizational policy in mind before attempting this method of getting access to your users emails. Following ways to you can try -
Google Apps Vault - Google Apps Vault is an add-on for Google Apps that lets you retain, archive, search, and export your organization's email for your eDiscovery and compliance needs. Vault is entirely web-based, so there's no need to install or maintain any software. Google Apps Vault provides users email and chat archiving, Legal holds, users Drive file search, users email search and export audit report.
Admin can easily write their different criteria and can find users email copy. For example I want to find Mr.xyz user emails with file attachments only then just write user name & choose appropriate option. Result will display all attachment file emails with of particular users. Like that more criteria are available in Google Vault.
Google Apps Dashboard Gmail Content Compliance - In Google apps get Gmail advance setting where you can easily put your own rules. With this content compliance setting you can get copy of each received and sent email of all users. Not only admin but also you can setup such rule for any users. But before doing any setup take knowledge about it. After setup Google forward users copy to you when he sent or received any email from your domain or outside the domain.
The Content compliance enables to specify what action to perform for messages based on predefined sets of words, phrases, text patterns, or numerical patterns. The content compliance setting scans messages for content that matches one or more rules that you configure within the setting. Admin can choose whether messages are rejected or delivered with modifications;
Automatically Forward email to Another email - This is very easy but time consuming process. To auto forward email to another email admin has to make setup in every opening users accounts. In Gmail account setting just click to Forwarding option and follow the instructions. Admin has to put his own email id in Auto forwarder option. But only users received copy can get admin. It applicable only for incoming email of user. You can forward all of your new messages, or just specific kinds of messages to specified email id. Note that spam messages aren't included when you choose to forward all new mail.
Some of following solution will only work with Business, Education and Government edition of Google Apps, and not with free edition
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