Airmail adds the much awaited DEVONthink integration which allows you to send a message straight into the DEVONthink Inbox.
How to enable the DEVONthink integration?
You can enable the integration in Airmail Preferences > Services as shown in the screenshot below
How to send a message to DEVONthink?
To send a message to DEVONthink, select a message in the message list and then press 'z' to open the 'send to' context menu as shown in the screenshot below
How the message will display in DEVONthink?
The message will be sent to the Inbox of DEVONthink with the callback link of the message as shown in the screenshot below