Mail (Yosemite): Address an email message
You can use Smart Addresses to make it easier to address your messages.
Use individual addresses
Type names or email addresses into an address field (such as To). Mail tries to complete addresses as you type them, based on people it finds in the Contacts app and the Previous Recipients list, and on any available network servers.
You can also click Add 
in an address field, click a contact in the list, then click an email address for the contact.
To prevent recipients from seeing each other’s email addresses, put the email addresses in the Bcc field (you can leave the To field blank). People receiving your message see “Undisclosed-recipients” in the To field.
If you don’t see the Bcc field, click the Header Fields button 
in the toolbar of the message window, then choose Bcc Address Field.
Use a group address
To prevent individual email addresses from being shown when you send messages to groups, choose Mail > Preferences, click Composing, then deselect “When sending to a group, show all member addresses.”
Set the From or Reply To address
If you use several email accounts in Mail, your messages are sent from the account of the currently selected mailbox. However, you can change which account is used to send messages. You can also specify an address where you want to receive replies to your message.
Change the account for the current message: Move the pointer over the From field, click the pop-up menu that appears, then choose an email address. If you set up email aliases, those are also shown. For some accounts, you can choose or change the outgoing mail server.
Use the same account for all messages: Choose Mail > Preferences, click Composing, click the “Send new messages from” pop-up menu, then choose an email account.
Set a Reply To address: Click the Header Fields button in the toolbar of the message window, choose Reply-To Address Field, then enter the address where you want to receive replies to your message.
Highlight unintended addresses
You can have Mail display email addresses in red if they don’t match “safe addresses” that you specify. You might want to do this, for example, to help prevent accidentally sending work email to people outside your company.
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Choose Mail > Preferences, then click Composing.
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Select “Mark addresses not ending with,” then enter one or more email addresses, separated by commas. To enter just a domain (such as “apple.com”), include the “@” symbol (as in “@apple.com.”)
Email addresses that match the specified addresses or domains appear as usual when you address a message; email addresses that don’t match appear in red.
You can drag addresses between address fields and messages.
Some mail servers won’t send a message if even just one address is incorrect. Try to remove or correct invalid addresses, then send the message again.
Mail uses email addresses that are in the Contacts app. You can import email addresses from other email apps into Contacts, to make the addresses available in Mail.