Suddenly the best email application ever (Eudora) stopped being supported by Comcast. So I've been forced to wrestle and grumble with Mac Mail.
I feel your pain—If I had one salient point to make, it would be—it never pays to get too far behind in the hardware or software.
I transitioned from Eudora on 9/19/2012. I tried every email client alternative out there over the course of 6 months, including the open source Penelope.
I came to realize Mac's Mail.app does everything I need to run smoothly my 4 separate businesses and 12 email addresses. I did many test runs before I fully transitioned myself to the Mail.app It was important to retain all email historic records going back to 1998 with as little loss as possible.
I have not been disappointed once in 8 years using the Mail.app without fail.
In 2006 Qualcomm stopped development of the commercial version, and sponsored the creation of a new open-source version based on Mozilla Thunderbird, code-named "Penelope," later renamed to Eudora OSE. Development of the open-source version stopped in 2010 and was officially deprecated in 2013, with users advised to switch to an alternative mail client..