The Main tab contains mostly contact information. You can enter multiple phone numbers and an address, save a driver's license number, and indicate groups to associate the customer with.
Email
Clicking Send Email allows you to send an email directly to this customer. You can request an email acknowledgment, set the priority, and attach a document or image using the paperclip.
Show Emails lists all email communications with the customer.
Note that email must be enabled in System | Company Setup | Company Information | Mercury Server Settings in order to email customers.