Airmail 3.5.3 (461)
MacBook Pro running OS X 10.11.6 (15G17023)
Preferences->Appearance as shown in first attachment

Problem: text wrapping in Message Detail pane truncates message text on right margin, but only after Message Details have been turned off and then back on. Restarting Airmail fixes the problem. I have replicated the problem and show the steps to make it appear.

1. Figure Detail-Window-Normal.png shows the message detail window behaving properly - right edge wraps text in the message

2. Now close the Message Detail pane, either with command+left-arrow or using the View->Hide-Message-Detail.

3. Re-open the Message Detail pane using either the keyboard shortcut or menu.

4. Figure Detail-Window-Truncated.png shows that the Message Detail view now truncates the right side of the message by a few characters. This is more or less noticeable depending on the message.

5. Exiting Airmail and restarting it restores the correct behavior.

Paolo