To customize your Auto-correct settings go to File > Options
Figure 1: File, Options
Navigate to the Mail > Spelling and Auto-correct button
Figure 2: Outlook options dialog
Select Auto-correct Options. Note: There are several other Proofing options on this screen. Please take note and adjust should you desire.
Figure 3: Outlook options dialog
You will be presented with the Auto-correct settings.
Figure 4: Auto-correct options
For this example we will opt to replace msg with message. This will enable to use this abbreviation when composing a message, and allow Outlook to fill in the entire word for us.
Figure 5: Auto-correct options