If you own, or have access to, an FTP server, it’s easy to use Airmail to automatically upload and store your message attachments there. First, go to any “Preferences” page and at the top-right, click on “FTP” which will present the screen below.
Add your complete server path to the folder from where you want to attach the attachments, your Username and Password, then sign in. Airmail will remember your settings (but for your privacy not retain them).
This is similar to choosing Dropbox, Google Drive, or Evernote for the same purpose of storing attachments from your email.