Many Tor users don’t use stream (curcuit) isolation at all.
Not every mail client have support for it, but Delta Chat apps have it, so it would be a shame not to use it.
For using Tor stream isolation in current versions of DC apps, set proxy URL in form of socks5://user:pass@127.0.0.1:9050 for each mailbox account.
IP address and port (127.0.0.1:9050 in my example) may be different on your system, but usually should be the same for all your accounts on that system.
user:pass may be almost any string with : somwhere in the middle, unique for each mail account.
Example:
socks5://a:01@127.0.0.1:9050- account #1, work chatssocks5://a:02@127.0.0.1:9050- account #2, family chatssocks5://a:03@127.0.0.1:9050- account #3, chat with very secret soulmate
and so on.
How it works: Tor router’s SOCKS5 proxy will not actually check those user:password pairs against some database; it will use it in hashed form as a tag for marking routes that it builds through Tor network. In ideal conditions, every such tag will get it’s own route end exit node, so it will be more difficult to tie your mail accounts together.
For further reading I leave here old link from my bookmarks, but I’m sure there are better explanations available.