If like most people you have a letterbox, it could make you more vulnerable to burglary. My advice would be if you can afford it, or when you replace the door, loose the letterbox altogether and put an external mailbox on the wall outside.
Letterboxes firstly enable a potential burglar to look into your house and also listen to see if you are at home. If you keep house or car keys anywhere near the door some burglars will use a rod to steal the keys to either gain entry to your house without damage or more likely just steal your car.
If you have a UPVC door or other type with multipoint locking, some might allow a burglar to open the door simply by using an implement to pull the internal handle down or turn your thumb-turn cylinder.
You can prevent the handle being pulled down simply by pushing it up and locking with the key. It has been known for insurance companies to question whether a premises was properly secured when doors have been opened through the letterbox.
So what can you do to make your letterbox more secure?
Luckily there are a few simple steps you can take, you could fit a security cowl to make it harder for a burglar to reach your lock, you can also fit a letter cage or fireproof security letterbox. There are many different types to chose from, your local reputable locksmith will happily advise as to what would work best for your type of door and situation.