Sending messages from an iPhone sometimes goes to email rather than to mobile

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.

Solution 1:

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.