Outlook

Try using this - https://docs.appian.com/suite/help/22.1/Task_Viewer_Add-in_for_Microsoft_Outlook.html

It will work if you have SSO enabled 

I assume that doesn't work because you don't want to just avoid the login screen, you want to avoid the recipient even having an Appian account at all.  Unfortunately, there's very few ways for an unauthenticated user to interact with Appian.

Portals is one way, and I don't know if it would require additional licensing.

Another approach might be to have the "Approve" or "Reject" buttons configured to send emails.  The email address could be set to an Appian process model, which will kick off when it receives the email.

I assume that doesn't work because you don't want to just avoid the login screen, you want to avoid the recipient even having an Appian account at all.  Unfortunately, there's very few ways for an unauthenticated user to interact with Appian.

Portals is one way, and I don't know if it would require additional licensing.

Another approach might be to have the "Approve" or "Reject" buttons configured to send emails.  The email address could be set to an Appian process model, which will kick off when it receives the email.

Another approach is to add a script in the HTML code to call a webAPI on Appian using a service account. Not a good one tho, but can work if there is a dead end. 

Thanks for sharing the details.

I am not sure if there is a way we can set an email address to trigger the Appian process model.

https://docs.appian.com/suite/help/22.1/Sending_an_Email_Message_to_a_Process.html  It looks like it's a bit complicated, and I have to say that I've never personally done it myself, though I have seen it work on projects I was on.