Rules can be configured to apply to all users or you can limit the scope of senders using various criteria.
Use the Azure AD field filter if you want a signature to be added to an email only if the sender matches very specific criteria.
In the Design tab, you can access the signature template editor to create a signature or you can load one of predefined templates.
CodeTwo Email Signatures for Office 365 comes with a WYSIWYG editor that lets you easily design email signature templates in HTML, RTF and Plain text formats.
You can also turn on a feature called Sent Items Update, which lets you display email signatures in end-users' Sent Items folders.
The program automatically integrates with your Office 365 tenant - no manual configuration is needed.