To add an account, go to settings, click on the “Accounts and Import” tab, and click “Add a POP3 mail account you own” in the “Check mail from other accounts” section. (Check Gmail’s help docs if you have any trouble with adding a particular account.)
Now, you can archive emails from different accounts in one place, use one labeling system for all of your accounts, and even respond to emails using different profiles.
If you’re worried about staying organized, you can still search by the receiving address by using a to: address@example.com query, or by setting up multiple inboxes to show all account activity at a glance.
