To enable 'Do not Disturb' mode in Airmail, please follow the steps:
- Go to Airmail Preferences > Accounts.
- Select the account (for which you want to enable the Do not Disturb mode) by double clicking on the account and then choose 'More' tab.
- Then just go to 'Sounds' section and choose 'OS Default - Respect the Do not Disturb'.
This will make Airmail respect the Do not Disturb settings of your Mac and will not display any notifications.
Please note that this a 'per account' preference, therefore, if you want the same for complete Airmail app, you will have to choose this option for all the accounts separately.