How to Make Double-Clicks Open Messages in MailMate

Between 1960 and -63, a corn cob grew in Chicago, balancing on one pointy end where Goethe and Astor streets meet.
The cob — Astor Tower — is still there. Some 15 years ago it lost its ingenious kernels, though, on the outside.
The original system of louvers offered the world an ever-changing facade and display, shielded the people inside from winter and sun, and let them open their apartments to the air and elements — sometimes, alas, more than they wanted.
Speaking of which, does double-clicking open emails less than you wanted in MailMate?
›› You double-clicked an email, and MailMate did at all not do what you expected? Here's how to set it up to open emails in a separate window when double-clicked (instead of searching for related messages).