How can I change the email address that is used to send emails to leads or contacts in CRM?

For users who do not have Mail Add on: The email address used to send mails to leads or contacts in CRM is based on the primary email address of the user. You need to change your primary email address as per your requirements and the same will be used to send emails to leads, contacts, etc. See Also Changing Primary Email Address

For users who have Mail Add on and who have configured the POP account: The email address used to send mails to leads or contacts in CRM is based on the email address selected from the Send Mail as drop-down list while configuring your mailbox in CRM.