One common problem organizations face when sending emails to clients is that the email ends up in the Spam folder and the client misses out on their emails. Even though it is not completely possible to avoid Salesforce emails from ending up in the spam folder, but enabling email security compliance will help.
To do this: Navigate to Setup, under Administration Setup click Email Administration | Deliverability. In the Email Security Compliance section, make sure that both the “Enable compliance with standard email security mechanisms” and “Enable Sender ID compliance” check boxes are checked. The Sender ID compliance declares Salesforce.com emails as coming from an authorized IP address.
This setting will help to authenticate the emails, which in turn will prevent the emails from landing in the recipient’s spam folder.