Similar to my ed(1)→vi/vim post, today's #FreeSoftwareAdvent follows mail(1) & mutt/neomutt with appreciation (and minor lament) for Claws Mail and Thunderbird, too.
I used Thunderbird (and its Netscape Communicator suite ancestor before that) for a number of years in the late 1990s and early 2000s. It served us well as a standards-compliant IMAP/SMTP client for connecting to our email providers—both outsourced and after I started hosting our own mail-server. However, it started to get bloaty, consuming vast quantities of RAM, so I investigated alternatives and found Claws Mail.
I switched our household laptops over to using Claws and we enjoyed pretty much all the same benefits that we had with Thunderbird without the massive RAM consumption. Until it started consuming RAM too.
So I eventually switched to mutt/neomutt for my own mail. But I still appreciate the time I had with Thunderbird & Claws.