I have a Retina macbook plugged into a DELL 24" monitor. I've noticed TWO anomalies using Airmail in this configuration:

1) Email attachments are never displayed on my non-retina screen. Just a huge blank space where the attachment should be.

Dragging the window across to my Retina then displays the attachment.

2) grabbing a screenshot on my retina screen and dropping into an email displays the image at double the resolution - I guess this is expected behaviour but it does mean that I'm forever resizing images and reloading them.

Robin