I want to monitor employees interactions inside companies. In the case the company is using Gmail, I was thinking about using https://developers.google.com/admin-sdk/email-audit/.
But i still have some questions regarding the "lawful" purpose and I'm wondering if Email Audit is the right API if my purpose is to monitor in real-time emails knowing there will be at least 10 000 emails/day to monitor.
If you check the Usage Limits and Quotas:
To answer you question, if your goals falls under this description - Google Apps Email Audit API Developer's Guide:
Then the answer would be yes, it is the appropriate API to use. If you are thinking about the 10000 emails/day, you might want to check if it is reasonable to ask for quota increase.
Hope this helps!