Hi @Andrew Kennard
Currently the Graph API does not support the use of signatures when creating a new mail draft, see the documentation for Creating a new mail draft:
https://learn.microsoft.com/en-us/graph/api/user-post-messages?view=graph-rest-1.0&tabs=http
we suggest that you can submit a feature request idea using this support link, which will be monitored by Microsoft team and make the enhancements to Microsoft Graph APIs.
Hope this helps.
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