Airmail can integrate with OneDrive. To enable the OneDrive integration, please go to Airmail Preferences > Services > One Drive and then click on 'Link OneDrive' as shown in the picture below.
You can save the files and create links on OneDrive (as .pdf files) and attach the files from OneDrive in the Airmail composer.
How to save a message to OneDrive from Airmail?
Please follow these steps:
1. Right click on a message and select 'Send to' or just press the 'Z' shortcut key to open up the share menu.
2. Then choose 'Save to OneDrive' as shown in the picture below. You can also configure 'Save to OneDrive' as a 'Menu Button'.
This will allow you to save it to your desired folder in OneDrive.
How to attach using One
You can attach using OneDrive in Airmail Composer as shown in the picture below:
Once attached and the upload finishes, a link will be created on your linked OneDrive account as shown in the picture below: