Michael Lucas has a long history of writing books about BSD and networking including Absolute FreeBSD, Networking for Systems Administrators, and SSH Mastery, among many others. He is working on his next book: Run Your Own Mail Server: A Book for Independence & Privacy.
Summary:
Running your own mail server is not only an act of defiance against some of the largest exploitative companies in history. It is not a mere education in protocols. Email is essential to modern industrial society. By running your own email, you seize control of your communications. You can tune your email to fit your needs, rather than accepting the defaults imposed by a company that exploits you without a speck of consideration for any of your issues. You own it. Running your own email requires only freely available tools, a server, and some knowledge. This book will give you that knowledge.
Voyager is definitely the most polished and great for those who used Apollo for Reddit. It is also very actively developed. For the Android users who do not mind the iOS aesthetic, Voyager is one of the few Lemmy clients on F-Droid.
Thunder is also a great option. I personally prefer its UX more than Voyager, but it is not quite as polished.