These instructions apply to version 4 of Airmail. For older versions of Airmail, please see the Setting Up Airmail 1–3 section.
Make sure that you are using 4.0 or later.
From the Airmail menu, choose Preferences….
Click on the Accounts icon at the top of the window. For each IMAP or Gmail account in the list:
POP accounts do not have the Download section, so you can skip this step for them.
Click the Save button.
Click on the Advanced icon at the top of the window and make sure that Use SpamSieve is checked.
Now it is time to train SpamSieve:
The Do an Initial Training section explains which messages (and how many of them) you should train.
On macOS 10.14 or later, the first time you try to train a message macOS will ask whether Airmail can control SpamSieve, and you should click OK.
SpamSieve will process new mail automatically. Messages that it thinks are spam will be moved to the Junk or Spam folder. Airmail will automatically launch SpamSieve when there are new messages that need to be filtered or when you train messages as spam or as good.
If you need to troubleshoot SpamSieve and Airmail, see the Checking the Airmail Setup section.