Using older versions of Outlook for Mac for personal email and calendar
Just a heads up, I've updated to the latest version available 16.69 (22123000) and attachments with digital signatures appears to be fixed. This is the first time I've been able to do both successfully.
EDIT: Although I discovered another bug where I couldn't actually delete the email afterwards lol. "Delete" was greyed out, even after restarting Outlook.