Quarterdeck Mail 4.2 Beta is now available. This version is Mac OS 8.1 compatible. However, it still has some unrelated problems, so be sure to go through the About QD Mail Client Beta file carefully before installing.
If you have added to your dictionary file, you should do a custom install to make sure that you do not replace your old dictionary. The About QD Mail Client Beta file explains this further.
Please note that you will need a Quarterdeck Mail Server 4.x to use the Quarterdeck Mail Client 4.2 Beta. The website has links to 4.1 Server demo software.