When you create a campaign, you can select to display a recipent's name in the To: field rather than their email address. In the To section of the Campaign Builder, you'll check the box next to the Personalize the To: field option. Outlook will almost always break the To: field if it's personalized and a subscriber doesn't have data for their first or last name.
If personalization is the culprit for your email showing as broken code in Outlook, you'll see one of two things in the To: field—quotation marks (" ") followed by the email address, or =?utf-8?Q??=.
To prevent this from happening, uncheck the Personalize the "To" field box in the To section of the Campaign Builder.
Any campaigns that are sent out with personalization turned off will display only the recipient's email address in the To: field.
If you want to enable personalization for the To: field but don't have first and/or last name values captured for every subscriber, you can set a default merge tag value to populate as a placeholder for missing information.