If you use Outlook Web App on a mobile device, tablet, or laptop, there is a separate process for creating or changing an email signature. A signature created or changed using Outlook for Office 365 will not transfer into the web app. Here’s how you change your signature in the Outlook Web App:

1. Select the settings menu in the upper right corner
2. Select Options
Under Mail>Layout, select Email Signature
(You can also search for ‘signature’ in the search box under the gear.)
3. edit your online mail signature office 365
4. Create your signature. The toolbar allows you to format your signature with fonts, colors, and size. You can also choose to automatically include your signature in each email at this time. Or, if you wish to manually add your signature each time, leave this box unchecked.
5. Click Save