It is possible to email directly into a matter in Actionstep however, this is changing.
As at 7 September 2021, you can configure Actionstep to use Subject Line Method or Wildcard Method.
Subject Line Method:
Add into the subject line of an inbound email sent to the email account configured for inbound with Actionstep id=?????:a???? where id=???? is the orgkey of your Actionstep system and where a???? is the matter number.
This allows you to email firstname.lastname@example.org and Actionstep is configured to file the inbound email on the matter number and copies the matter Assigned to.
Inbound email is handled via the 'POP Protocol' between your email server and Actionstep.
However, this POP protocol is being terminated by Microsoft and Google. The termination was planned to take place in October 2020 but due to Covid and the work involved globally by companies to change processes, it has been delayed for 12 months.
Earlier this year Microsoft has advised that they are doing a staggered termination. Read release notes from Microsoft on this point. This means if you don't have POP enabled on your email server, then it may already be disabled. However, you have POP enabled, then Microsoft will contact you at some point and provide you 12 months to stop using it before it terminates.
Once this happens by Microsoft both inbound options being Subject line and Wildcard will cease to work.
Actionstep doesn't have any other options for inbound email currently besides filing emails with your Outlook Add ins.
Here is the guide on the current Inbound email handling features - https://actionstep.zendesk.com/hc/en-us/articles/900004897306-Inbound-Email-POP-Collection