You can make Airmail as the default mail client for your OS X device.

To make Airmail your default mail client go to Apple Mail app preferences and set default mail reader as Airmail as shown below.

If the above process does not work, please follow the steps:

You can reset Apple's Preferences Database.

Open Terminal (Cmd+Space Bar and enter Terminal)
Copy/paste/enter this command:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -all local,system,user

Then set the default preference in Apple Mail again.
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