If you have another email account on a service that Gmail supports, you can use that email account inside Gmail. This allows you to see messages from both accounts together, as well as use Gmail's sorting and searching options. While not all email accounts are supported, most common webmail providers can be added to Gmail.
Once you’ve linked another email account, you have some settings you can adjust to change how linked accounts work.
Click the Settings icon.
Click the Accounts and Import tab.
After another account has been linked, some new settings will be available here.
The Send mail as section lists the accounts that you can send email as, with the default account labeled as default. You can click the make default link for another account to change the default.
Select a default reply behavior by selecting an option under the When replying to a message heading. You can reply from the email address that a message was sent to, or always reply from your default account.
Edit or Unlink accounts from the Check Mail from Other Accounts section.