Microsoft 365 server-side signatures will be enabled when the option Apply to email from all devices including mobiles is selected in the Signature 365 setup wizard:
When you enable this feature signatures are added to email after email is sent. This works for all email sent from any email client and device.
How it works
To inject signatures into email, Microsoft 365 is configured to send email via the Signature 365 service. The Signature 365 service injects signatures into email and then returns email to your Microsoft 365 tenant for delivery to the recipients. Because Signature 365 is hosted in Microsoft Azure same as Microsoft 365, your email never leaves Microsoft's data centres.
The email flow is illustrated below:
Your Microsoft 365 tenant is configured with an Outbound Connector (to send email to Signature 365), an Inbound Connector (to receive email back from Signature 365) and a Transport Rule (to route email via the Outbound Connector).
Your email is safe when using server-side signatures. Signature 365 is designed to meet the highest security, availability and compliance standards.
You can install the Signature 365 add-in for Outlook to enable signature previews in Outlook for Windows, Outlook for Mac and Outlook on the Web. When users compose email, they can see a preview of the email that will be added to their email right next to the email they are composing.
Sent items update
You can enable the Sent Items update feature to have Signature 365 update emails in Sent Items with signatures that have been added after the email was sent. You can also configure if you want sent items to be updated in place or a copy with signature created for each email.
Convert plain text email to HTML
Many mobile devices send plain text email by default. By default, Signature 365 will convert plain text email to HTML and add your HTML signature, but you can disable this feature if you want Signature 365 to just add a plain text signature to plain text email.