Airmail has a feature called 'Sending Delay' which is similar to Gmail’s 'Undo' feature. With 'Sending Delay', you can set your outgoing messages to get delayed by the fixed value of time before they get sent.
Go to Preferences > Composing > and select the desired value of 'Sending Delay'
Where to find the messages when they are 'Pending' due to the set delay?
If you want to stop a message from being sent that is still in the 'Pending' state for the time that you chose for 'Sending Delay', go to the 'Drafts' folder of the account from which the message was to be sent in the Airmail UI (see following screenshot) and click on 'Cancel'. If you click 'Cancel' the message will not be sent and will display 'Canceled'.