I like that you can send out bulk emails with multiple templates at once; it makes for easier workflows! There are also some more advanced features such as sending HTML email or customizing subject lines to make them look better/catch attention faster than standard plain text ones would do (which is great!).
You cannot create autoresponders though which could be useful if they released an add-on product but there's nothing wrong about not having this feature now either so no complaints here :) The only thing i dislike currently when using zairmail through my personal computer instead of gmails web interface - sometimes its just slow loading up certain pages especially those who have many images included inside their html code egspecially photos taken during our photo shoots / events etc.