iCloud: Send email using an alias
You can set up iCloud Mail to send messages from an alias email address.
For example, you can use your main iCloud Mail address with friends and family only and use an email alias for online registrations and purchases, joining mailing lists, and so on. This makes it easier to monitor the sources of unwanted messages.
Send email from an alias email address
Click the Compose button.
Type your recipient’s email address in the To field (separate multiple email addresses with a comma), and type a subject in the Subject field.
Choose an alias email address from the From pop-up menu.
This is the address that the recipient will see.
Type your message, then click the Send button in the upper-right corner of the New Message window.
You can also use OS X Mail to send messages using an alias. To do so, choose the alias from the From pop-up menu when composing a message.