MailSpring — Awesome Open Source
Suddenly in 2021 there appeared an open source desktop email program that I can recommend as a great choice for general users.
Open Source Email Apps
The main open source alternatives are Thunderbird and Evolution, both comprehensive in features, with lots of add-ons and integration with calendars and much more. On the down side they are both dated in design and clumsy to use, in comparison. On Linux, Geary is also worth a look.
I used Thunderbird for many years. I was happy with its power, and a bit frustrated with its clumsiness.
MailSpring is like Web Mail for Desktop
One substantial difference in behaviour is that Thunderbird and Evolution both work by downloading all your mail to your computer. Some advantages of downloading are that you can work off line or with a slow connection, and they can act as a (limited) kind of local backup of your mail.
MailSpring by contrast works by fetching the messages you view directly from your mail server on demand, like a web mail interface does. Advantages are that this makes it simpler, needing no options for choosing how to synchronise the local store with the server, or for having a separate local mail box, or for performing maintenance on the local store.
No connection to a shared address book
To help you auto-complete an address when you are writing a new email or searching for an old email, MailSpring seems to look only at the addresses in your mail history. That is useful enough most of the time.
I would prefer if it could also read from and save new addresses to the standard (CardDAV) online address book that I share with my family and with my other apps and devices. That would be more useful, because these days we use email addresses in many more apps than just an email program. There is a feature request for this.
I am finding Mailspring works pretty nicely for me as my daily desktop email app.