My first question would be why are you replying to emails from your Sent Items?

Secondly yes you are correct, because you are trying to "reply" to an email in your sent items, because you were the sender, it will default to trying to reply back to yourself so you either need to manually change the name in the To field or instead of replying, simply use the Forward option instead and enter the persons name. This however does not automate Outlook putting the original recipients name in the To field.

If you did want to do this from the Sent Items you could work around it by opening the email from the Sent Items list, from the Actions group select the Other Actions button and choose Resend message. The original person you sent the email to will already be listed in the To field but so will the original text you sent so you would just need to delete out the body text and type in your next message.