I have an iPhone 4S. When I send a text message it sometimes sends to the contact's email address rather than their mobile, if within the contact I have both email and mobile included. How do I prevent this - I always want to send to the mobile number. Also, I can't tell whether it goes to email addr or mobile number - the receiver tells me eventually.
When you create a new message, it will prompt you to chose between somebody's email and phone number to create a conversation (for MMS and iMessage conversation)
You probably chose the email by mistake once (and maybe the phone number another time), so now, you have 2 conversations for the same person. One to the phone number, one to the email.
You will need to delete the email one by opening the conversation, scrolling to top and selecting "Contact". From this view, you will see either the email or the phone number in light blue. If it's the email, go back and delete this conversation.
In the case where you are sending an iMessage, the only item you have control over is the address book entry.
If you delete the email address - your Messages app will be forces to ask for delivery to the phone number. If Apple messes up the delivery down the road when looking up the actual devices and indicating which thread / to address to actually send there's not much you can do.
The person on the receiving end has the most control over this. If the reason you are wanting to change this is for their benefit, you can get them to go into their settings and remove the email addresses.
Before Mountain Lion and iOS 6 you had to use an email address to "receive at" on non phone clients, but you can now de-select those email addresses and only use one (or several phone numbers) as only option for the "receive at" and "start new conversations from" settings.
The iOS preferences for Messages are not as complicated so I showed the more involved of the two. You can do the same deselection on all iOS devices and clear out any old conversations so you don't accidentally re-use the wrong address.