Anyone know of a program for a Mac that does the same thing as Mailwasher Pro, now that a Mac version of Mailwasher Pro is no longer supported? In other words, a mail program that allows you to preview your mail BEFORE it reaches your hard drive.

You have always been able to access, preview and delete your mail at your ISP. Mailwasher just streamlined that process.

I am not aware of another programme like it for Mac or Linux.

You can try to run the Windows version via Crossover Mac (from Codeweavers). I don't know if that works but I can verify that Crossover Linux runs the Windows version without any problems.