Hi!
As Microsoft still doesn’t incorporate the default Outlook signature when creating mails via MAPI, see Messages do not include default email signature - Outlook | Microsoft Learn, it would be great, if you could enable it when creating the e-mail.
All availble signatures and the default one you can find in %userprofile%AppDataRoamingMicrosoftSignatures and the registry. Here is a KB-article which describes some registry settings: https://support.microsoft.com/en-us/kb/2691977.
Also read this article, it describes another location where you can get the settings of the default signature: Configuring Outlook for the signatures within the users registry | ifnotisnull
On the backlog for LL25. We’ll probably additionally offer a setting for the path to the signature, skipping the question of finding it in the registry if given. That way, you’re not restricted to Outlook signatures but can use an arbitrary HTML/RTF/Text file.