You can be using company-provided email or third party email account. You can use Email to Salesforce to assign email to leads, contacts, opportunities and other records in the Salesforce. Enter the email in the BCC field when you are composing, forwarding or replying to an email. Salesforce receives a copy of the email and depending upon the configuration it gets added in to either the Activity History related list or My unresolved Items Page. Then you can assign them manually.
Setup > Email Administration > Email to Salesforce
Email to Salesforce
Active: Select to enable Email to Salesforce
Advanced Email Security Settings: (Optional) to verify the legitimacy of the sending server before processing a message.