When you set up the Messages app in iOS or OS X, you must supply it with an active Apple ID. But a single Apple ID can be associated with multiple phone numbers and email addresses, and the iMessage system enables you to send and receive messages to and from any email address or iPhone number thats associated with that ID.
That can have a lot of repercussions whether youre setting up the Messages app in iOS (on an iPhone or other device) or in Mac OS X 10.8 Mountain Lion. For example, on an iPhone, you could opt to receive only those iMessages that are directed at its phone number. At the same time, you could configure the Messages app on a Mac to receive iMessages at that same phone number as well as at a dozen different email addresses.
Starting with iOS 6 and OS X 10.8.2, when you are logged in with an Apple ID and you open the settings for Messages, youll see a list of every email address associated with that Apple ID. If you have an iPhone or iPhones, their phone numbers can be used with any other copy of Messages logged into the same Apple ID as wellthats how you can get iMessages sent to your phone number on your Mac.
To set up all of these options in iOS, you tap Settings > Messages > Send & Receive. On an iPhone, the phone number will be checked and grayed-out, and cannot be de-selected; you can only disable iMessage on the device. On all other devices, you see a list of phone numbers and email addresses that youve verified with your Apple ID. You can tap to toggle their use as destinations on that device, or tap the blue detail arrow to remove the address altogether; you can delete it from your Apple ID account, too, after you confirm that you want to do so. You add additional emails by tapping Add Another Email, then following a link embedded in an email message that will be sent to your Apple IDs main address. This leads you the Apple ID site, where you confirm the address.
In Messages in OS X, you choose Messages > Preferences, click the Accounts button, and select the iMessage entry. Log in to the appropriate Apple ID, if its not already connected. A list of email addresses appears under the You Can Be Reached for Messages At label. Toggle those to be included or not. Click Add Email to follow the same path as with iOS to add another address to your Apple ID account, and thus make it available in Messages to use with iMessage.
As of iOS 6 and Mac OS X 10.8.2, whenever you remove or add email addresses or phone numbers to Messages, or toggle an address or number off or on, every other piece of hardware associated with that Apple ID for iMessage pops up a notification informing you of the change. An alert also appears when you add an address to your Apple ID or activate iMessage on an iPhone. It can often be a case of too much information.
If you have an iPhone and your number doesnt appear as an option to receive iMessages in OS Xs Messages program, toggle the Enable This Account checkbox. If that fails, log out of your Apple ID account (click Sign Out) and then log back in. If that fails, too, in Settings > Messages toggle iMessage from On to Off and On. Finally, one more trick: Tap Send & Receive, then tap the Apple ID button at the top, and finally tap Sign Out. Log back in, and your iPhones number should (at last) appear in the list of available receiving addresses in OS Xs Messages.
As I said, you can opt to receive messages at a different set of addresses on each device you use. For instance, my old .Mac @mac.com address was converted to an @me.com and now to an @icloud.com address. I dont particularly need to use all three, so I leave everything but the @icloud.com address unchecked. You may also opt not to receive iMessages sent to an iPhone number on all of your devices, lest incoming messages light up all of your devices and your Mac at the same time.
As for choosing which addresses to use, there are a few strategies you could follow.
First, go narrow: Choose a single address or phone number to hand out to everyone. Consolidate all your iMessage delivery from colleagues, friends, and others to a single address or phone number that you enable on every device. That can be a problem if people have multiple addresses for you, because Messages in iOS and OS X let people attempt to contact you at any email address or phone number. Further, OS Xs Messages shows a blue iMessage icon next to any email address or phone number associated with an Apple ID or iMessages account, whether its set to be received anywhere.
Second, go wide: Pick a common set of addresses and numbers where you can be reached. Select (iOS) or check boxes next to (OS X) all of the most common ways people reach you. This should ensure youll get iMessages as you need them everywhere.
Finally, go specific: Set up a different set of delivery options on each device. You might want to use your phone number only with your iPhone, and different combinations of addresses and numbers on your various other devices. You can then ask those who use iMessage to reach you to pick specific methods to find you. That can become quite complicated, but its always an option. The choice of phone number on an iPhone and email addresses on other devices is an easier split, of course.
(Many email services let you set up multiple addresses that point to a single account; some support the plus-sign (+) addressing scheme. The latter means you can address an email to firstname.lastname@example.org, and it will be delivered to email@example.com but also passing along the +extra string to mail headers, where it can be used for filtering or other purposes. Thus firstname.lastname@example.org and email@example.com should be treated as two distinct addresses in setting up an Apple ID, but both would deliver to the same firstname.lastname@example.org mailbox.)
So far, Ive discussed the receiving end. But what about choosing an address to be used when you send an iMessage? The solution lies in a simple option: Start New Conversations From. This choice appears in the same form in both iOS and OS X. You can select among available contact methods, including a phone number connected to an iPhone and email addresses, to choose the way in which Messages handles new conversations. (This option was confusingly labeled Caller ID in iOS 5 and versions of OS X before 10.8.2.)
You can opt to send from any address or telephone number associated with your Apple ID. Prior to iOS 6 and OS X 10.8.2, Apple used whatever you selected for this option as the return address for new conversations, but it also used it as the snap-back-to address if someone from whom youd previously accepted iMessages initiated a conversation. Thats changed, thankfully: Whatever addresses you and an iMessage partner are using remain in use for the conversation. (You can click the arrow next to a contacts name in OS Xs Messages to change the method or address for a conversation, but thats a whole other issue.)
While managing addresses for iMessage is still sometimes confusing, its still more rational now that it was in earlier releases of iOS and OS X. Apple has made it more consistent and predictable to know where and how you send and receive iMessages.