In Airmail you can prioritise messages for your receiver which means the high priority marked as important mail and warns the receiver to pay attention as well there are different level of priority which are Normal, High and Low the choice is yours and you can decide whichever you would like to choose Airmail will perform it and warn the receiver with priority.
However prioritising messages does not effect the spam system, If you have your email listed as spammer your mails will not considered as prioritised sender which means your mail will be forced to move to the Spam folder and make sure that you don’t have your email marked or reported as spammer by the receiver.
Following the image below it explains you to choose priority for mails that you want to do so:
You can see the icon above as a 'flag'.
Please remember to enable it here: