Office 365 administrators can create email signatures for members of their organization via the Exchange admin center. This often-overlooked process actually involves creating an HTML signature and then applying it to outgoing messages automatically via a Transport Rule.

You can even include customized data for each user by including attributes surrounded by percent signs, i.e. %%Display Name%%.

The video above provides a step-by-step guide for creating organization-wide email signatures in your Office 365 tenant using a transport rule. All you need to do is follow these steps:

1. Prepare the HTML for your email signature and save it in a text file. For formatting tips, check out this TechNet article. If you’re including an image, make sure it is hosted where recipients can view it.
2. From the admin center, select Exchange to get to the Exchange admin center.
3. Select rules under mail flow.
4. Click the + button and select Create a new rule…
5. Add a name that you will remember e.g. HTML Signature Rule.
6. Click More options… at the bottom of the window.
7. From the Apply this rule if… drop-down menu, select your desired conditions. For example, you could select The sender is located… and Inside the organization. 
8. Under the Do the following… drop-down menu, select Apply a disclaimer to the message… and select append a disclaimer.
9. Click the Enter text field.
10. Copy and paste your HTML into the text box and click OK.
11. Click the Select one… field to enter a fallback action e.g. what should be done if the disclaimer can’t be added. For example, you could choose Ignore, so that the email will still go through as normal even if the signature cannot be added.
12. Add an exception if desired.
13. Click Save.
14. Send a test email to yourself (as long as it won’t be excluded by an exception) and make sure your signature looks and behaves exactly how you want.