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 one or more names or email addresses in an address field. If addresses don’t appear enclosed in blue fields, separate them with commas.
iCloud Mail tries to complete addresses as you type them, based on addresses in iCloud Contacts or recipients that you previously emailed. If iCloud Mail finds more than one match, it displays a list of addresses that match what you typed so far. To use one of the suggested addresses, select it, then press Return. iCloud Mail encloses the address in a blue field.
To view all of your contacts, click the Add button at the right end of the address field.
In the Subject field, type a subject for the email.
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 Send in the upper-right corner of the iCloud Mail 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.