It is possible to get Mailwasher Pro 6.3 running in Linux using the following:
Ubuntu 8.10 "Intrepid Ibex"
Mailwasher Pro 6.3 -- a Windows only program that has a cost. Used for pre-checking email accounts before download to email application.
Thunderbird 2.0.0.17
Wine 1.1.10

Wine fools Windows applications into thinking they are running on Windows.
I am not going to go into the method for getting Wine onto your Ubuntu. That can be found here:
http://www.winehq.org/site/download
Configure Wine.
Install Mailwasher Pro 6.3
Start it using the Wine menu entries.
Configure it to suit. LEAVE the email application settings for the moment.
Use the File menu to exit Mailwasher. The top-right X results in numerous crashes.
In Wine's Configure Wine applet, make sure a drive is set to /
and another to your Home directory
eg E: /
Z: /home/cw
Open Text Editor from Accessories menu.
put in:
e:\usr\bin\thunderbird
Save it to your home directory, where ever you like, call it tbird.bat
eg ~/Apps/tbird.bat

test the bat file:
wine cmd ~/Apps/tbird.bat

Ok, now to configure Mailwasher:
Open it again, go into it's settings,in General Options, click the first Specify button.

set the "specify a mail client command" to Z:\Apps\tbird.bat

Test launch. Close Thunderbird.
Click Ok.

Don't forget to close Mailwasher when you have finished processing the mail BY USING THE FILE MENU. File-Exit.

I still get Access violation at address 0040CCA9 in module 'Mailwasher.exe'. Write of address 00000000. OK.

Don't know why yet, but it's fewer errors than the other method to close, and doesn't result in any issues.

Also check out WinePath to help with the handling of Windows vs Linux paths.

Thanks to arethusa and Romster on #winehq channel at Freenode IRC.