Mozilla Mail doesn't do grouping, not that I can see in 1.3 anyway. I've checked it three times, and tried to see if there was an option for groups. I did not see one.

Eudora may be different, I haven't used it in a while. The free Version of Eudora, shows advertisement, you have to pay to make the ads go away. I'll download it and check it out when I find the time to do so. [link|http://www.eudora.com/|http://www.eudora.com/]