Send mail from multiple accounts that are the same account but belongs to different domains
Source: https://support.apple.com/kb/PH22279?viewlocale=en_US&amp;locale=en_US
Create or change an alias

Choose Mail &gt; Preferences, then click Accounts.

Select an account, then click Account Information.

Click the Alias pop-up menu, choose Edit Aliases, then do one of the following:


Send a message from an alias
In a new message, move the pointer over the From field, click the pop-up menu that appears, then choose an email alias.