(1) On the left menu, click Compose to go to the [Compose] page.
(2) In the [Compose] page, open the Tools drop-down menu on the toolbar and select Preferences.
(3) Locate the Default Sender setting. Open the drop-down menu to select the account you intend to use as your default sender account.
(4) Click OK to finish. All changes will be applied on your next login.
Before setting the Default Sender, please add a POP3 accounts first. For example, if you want to select firstname.lastname@example.org as your default sender, you have to add this account in your POP3 accounts.
You can also select the sender when composing the mail by click the From button.
3. Retrieving and Sending Mails-Advanced