When you accidentally enter overtype mode, your Microsoft Outlook 2013 program replaces letters as you type instead of moving them one position to the right. Overtype mode is not an error -- your keyboard is not broken and Outlook works as it should. The Insert key is responsible for switching between overtype and insert modes; you've probably pressed the key accidentally.
Deactivating Overtype Mode
In overtype mode, new letters replace existing letters instead of moving them to the right -- newly typed text appears to "eat" what's already present in your email message. You can accidentally enter overtype mode by pressing the "Insert" key. To switch to insert mode and prevent Outlook from replacing text, press the "Insert" key. On some keyboards, it may be labeled "Ins."
Overtype mode can be helpful in certain situations. It allows you to replace words without selecting and deleting them in advance -- you just click before the word and start typing. Some forms contain lines made up of underscore characters; typing on these lines moves them to the right and often often makes them wrap on to the next line. In overtype mode, each new letter replaces an existing underscore character.