This guide is only applicable after you completed the Server-Side (Rerouting) configuration.
The email signature is applied when the email is sent.
It's visible to recipients - Not visible as the email is composed.
You must use Outlook App for iOS or Native Mail app for iOS.
Outlook for iOS
Here is the email preview before sending.
'Get Outlook for iOS' is the default signature for Outlook app so leave as is.
Any signature will get replaced as the email is sent - Even empty signature will get a signature.
Finally, this is how it looks at the recipient's end where 'Get Outlook for iOS' got replaced:
Mail for iOS
Below is the default Mail App signature which is 'Sent from my iPhone'.
NOTE: Since Mail app sends messages in Plain Text format by default, the only default text strings will get replaced when sent. Learn how to change to HTML.