If, for example, you get a message in your e-mail that an invoice cannot be sent to a customer e-mail but cannot find the customer e-mail in the customer list, the email may be in the contacts list.
For quick searches of all e-mail addresses and other information in the customer, prospects, vendors and contact tables you can use the 'Create e-mail list' table under CRM instead of searching all the separate tables individually.
Go to CRM/Create e-mail list.