Airmail now allows you to use 'Do Not Disturb' feature to stop receiving notifications during the defined period. This is an 'Individual account' feature, therefore you can choose to use this feature only for the accounts you want.
To turn on this feature, please follow the procedure as explained below.
First, go to Airmail Settings as shown in the picture below.
Then choose an account for which you want to enable 'Do Not Disturb' feature.
Then choose 'Account Notifications' from the list of the settings as shown in the picture below.
Then choose 'Do Not Disturb' as shown in the picture below.
Now you will be taken to the settings of the 'Do Not Disturb' as shown in the picture below, where you can define the settings as per your requirement.
You can always turn ON/OFF with the switch provided in the same location (at the top) as shown in the picture above.