Pass tasks to Todoist in the form airmail://message-ID(Message subject line)
The current integration results in tasks being created in Todoist with a long link back to the message in Airmail that is visible in the task's description. Using the format airmail://message-ID(Message subject line) would result in 'hot' (clickable) task descriptions made of the message's subject line only which, imho, makes things easier to visualize and work with in Todoist.

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stan commented
there is a workaround:
check this site: http://thesweetsetup.com/add-tasks-todoist-email/
simply put: in todoist: create a link to mail to a specific project (or the inbox in general). Use that link in Airmail to create a custom action: forward. as fowarding adress, use the link from todoist. Done