1. Log in to the Office 365 portal using an Exchange Online administrator account and access the Microsoft 365 admin center as shown in the below image:

    configuring organization wide email signatures in office 365
  2. Expand Admin centers and click Exchange.

    Office 365: Accessing Exchange admin centerFig. 1. The Exchange option in the Microsoft 365 admin center menu.

  3. In the Exchange admin center click the mail flow option in the navigation pane (marked red in Fig. 2) and make sure that in the work pane the rules tab (marked yellow in Fig. 2) is selected.Exchange admin center: Accessing mail flow rulesFig. 2. The Exchange admin center page with the mail flow rules interface visible.

  4. Click the Plus icon and select Apply disclaimers… (Fig. 3).

    This way the subsequent new rule popup window will already have the Apply disclaimers action selected (clicking the Create new rule… option opens the new rule window without any preselected attributes).The list of available Mail flow rules' actions, which includes the Apply disclaimer action
    Fig. 3. The list of available mail flow rules’ actions, with the Apply disclaimer action marked red.

  5. In the resulting new rule window type in the name of your rule and configure the settings according to your needs.

Click on ENTER TEXT and add below HTML Code

<div style="font-size:9pt;  font-family: 'Calibri',sans-serif;">

         <img widht="100px" height="100px" alt="Brio"  src="https://briocoinws.appspot.com.storage.googleapis.com/BRIO_LOGO2000x1000.png">


      <span style="font-size:12pt;  font-family: 'Cambria','times new roman','garamond',serif; color:#ff0000;">Test Disclaimer Title</span>


      <p style="font-size:8pt; line-height:10pt; font-family: 'Cambria','times roman',serif;">This message contains confidential information and is intended only for the individual(s) addressed in the message. If you aren't the named addressee, you should not disseminate, distribute, or copy this e-mail. If you aren't the intended recipient, you aren'tified that disclosing, distributing, or copying this e-mail is strictly prohibited.  </p>

      <span style="padding-top:10px; font-weight:bold; color:#CC0000; font-size:10pt; font-family: 'Calibri',Arial,sans-serif; "></span>





Next, select the preferred fallback action, which will be executed in case of problems with inserting the signature. The choices are: wrap (forward the original message as attachment and stamp the forward), ignore (send the message without a signature) or reject (don’t send the message). 


Refer the link for your reference below: