1. When adding a signature go to the settings (it’s a  gear icon at the top right-corner of the screen) and click Mail(Office 365) 

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2.  On the left pane, extend the Mail section,  

  then Layout and select Email signature. 

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3. To make signature appearing in every new email message, check the Automatically include my signature on new messages I compose box. Then, compose your email signature using available formatting tools.

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4.Once your signature has been created, hit the Save button so that all changes take effect.

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5.Go back to your main Mail view and click the plus icon next to the New button or click the arrow and select Email message. 

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6. Your signature should be already applied to a newly opened message. 

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