Hi @Niko Vrdoljak ,
Currently with Microsoft Graph API, doesn't have support for Signature.
Refer to documentation mailboxsettings, which lists mailbox settings that can be managed through Graph.
There is Graph Feed Request submitted, you can upvote the request and provide you feedback, and Microsoft Product Engineering team can review and check the feasibility and consider for implementation.
Make email signatures available on Graph
Hope this helps.
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