Cashbook - Go to Help > Email
Stockbook - Go to Setup > Email Options
The settings for some of the common email providers are listed below
If your email provider isn't listed, please contact the Practical Systems Support Team for assistance.
Send yourself a test email to ensure the settings are correct.
Note that you may still have to modify settings on your Gmail account, depending on whether you are using 2-step authentication or not, and whether you have enabled the highest level of access security or not. For example, try the following web address: https://www.google.com/settings/security/lesssecureapps