Hi @Iva Kupecká ,
According to my research, 'digital signature' feature is not yet available in New Outlook for Windows.
We have identified this to be a known problem in the product. The product team has been made aware, and they are actively working on it. At this point in time, we have no final date available as it is too early in the process. Hence ETA is still unknown.
As this is an early preview of the New Outlook, not all functionality from Classic Outlook for Windows is available yet. We are working on adding more aspects, for more details you can refer to the link: https://techcommunity.microsoft.com/t5/outlook-blog/things-to-know-about-the-new-outlook-for-windows/ba-p/3383964
If any of those limitations are critical for your work, we recommend you stay in the Classic Outlook until they are resolved. You may switch the toggle "New Outlook" off (top-right), to revert to Classic Outlook.
(Important Note: If you do not see the 'toggle on the top right of your screen', please follow the steps mentioned under the section 'Going back to classic Outlook for Windows' given in the article link below.)
https://support.microsoft.com/office/toggling-out-of-the-new-outlook-for-windows-preview-ec102b39-5727-418e-ae1f-a1805434640c
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