Email delegation is a feature, enabled by Google. It will allow you to easily manage numerous accounts without signing in and out of those Gmail and switching accounts manually.
Originally, e-mail delegation was useful for granting others access to your primary Gmail account — personal assistants, for example.
Now, this feature is going to be more useful for any Gmail user with their multiple accounts. When you sign into your primary Gmail account, you can choose to grant access to another account. Just navigate to your Gmail settings by clicking the link in the top right corner of Gmail’s web interface.
Under the Accounts tab, there’s now a new section entitled, “Grant access to your account.” Here, you can add any other Gmail accounts you control to your primary Gmail account.
When you add an account, you’ll have to accept access from a verification e-mail sent to the to-be-added account. Once the account is successfully added, you can simply toggle between your Gmail accounts without logging in and out.
Also, when you send a new e-mail message while signed into a secondary account, your primary address will also appear in the e-mail details.