Users who are running the desktop version of Outlook may experience this if they haven't set up their email signature in Office 365 online.
Create email signature in Outlook 365
1. Sign into your Microsoft account with your username and password.
2. Click Settings cog button > View all Outlook Settings.
3. Click Compose and Reply
4. Under Email signature, type your signature and use the available formatting options to change its appearance.
5. Click Save.