your best email client?

Years ago there was TorBirdy, an addon for Thunderbird developed by Tor Project. It has been abandoned for years now.

From what I understand, the only "safe" way to use an email client (considering there are many other aspects to consider, try reading the old TorBirdy documentation) would be to use TAILS.

Personally, I would say use Tor Browser and a webmail client.
danwin1210.de has Squirrel mail and an onion service, which doesn't even require JS. It's a good bet, as good as any other.

Icedove endures everything you mentioned as a requirement. And is installed by default in Trisquel.

in the claws mail menu there is no option to select proxy dns. Does claws mail not use dns?

I am not sure what you are looking for. Every email client uses DNS, but this is more a system setting than a per application setting. If you want to use a particular DNS server, you can configure it in your network settings, whether you use MATE, KDE or another desktop environment.

I use GNOME evolution mainly. Claws mail is much lighter (and is very good), evolution is similar to icedove. One reason why I use evolution rather than icedove is because evolution uses GPG keys from the keyring directly, while icedove wants to store them separately. I also tend to find evolution easier to use and configure, but it may be subjective.

If you want a given program to access the internet over tor, then you have to configure that program to use tor and depending on the program also make the program use a proxy dns?

Honestly, I don't know. From https://daniel.haxx.se/blog/2024/05/17/curl-tor-dot-onion-and-socks/, I see:

You know Tor, but do you know SOCKS5? It is an old and simple protocol for setting up a connection and when using it, the client can decide to either pass on the full hostname it wants to connect to, or it can pass on the exact IP address.

From further reading, with curl, it is possible to decide whether to use tor to resolve host names (so no need to use a DNS server) or a DNS server. Perhaps which way it happens is decided by each program. For claws-mail, the FAQ for using Tor says to just set the proxy (see http://web.archive.org/web/20231123082117/https://www.claws-mail.org/faq/index.php/Connecting_with_Tor, using wayback machine now because the site does not respond).

From https://wiki.archlinux.org/title/Tor#Using_TorDNS_systemwide, I see that it is possible to always use tor to resolve hostnames but it is likely to be slower.

privoxy + icedove is a good way to go.

also privoxy + web browser as well.

Also, not that everyone would approve of this idea, but if you need a more up to date icedove, even if its based on an older toolkit, g4jc gave me patches to build icedove-uxp.

I don't expect this to be enough for everyone, but send a reply whenever and I can see if I can build from latest source then I will get back to you.

You can also pm me btw if needed.