I believe there are some plugins that support similar features, but I have not yet found a Mail.app snoozing that is portable as well (eg: Android/webapp support).
It can also be pretty laggy (e.g. selecting multiple messages), but this has gotten a lot better in High Sierra.
I still use it though since I have yet to find an alternative that I prefer (I've only tried a few though)
https://www.reddit.com/r/privacy/comments/5grsan/do_not_use_...
The obvious tradeoff the relative lack of polish and overhead during initial setup, but if you're willing to put up with that it's a fine choice.
The author also maintains a blog[2] where he post quirks about several IMAP implementations. For example, Yahoo IMAP[3] or Gmail labels[4]. The app itself also handled all these edge cases very well.
[1]: https://freron.com
[3]: https://blog.freron.com/2017/slow-descent-into-madness-yahoo...
[4]: https://blog.freron.com/2013/mavericks-gmail-apple-mail-and-...
Also has the best "less important emails" inbox implementation that I've used so far. The whole gimmick of an AI bot I could chat with is just that, a gimmick, but bot the desktop and mobile apps are solid.
Wishlist items remaining: Newton had you have a Newton account that would automatically set up all your accounts when you installed it on a new device, but Astro doesn't do that. Also, I'd love it to automatically pick the right account to send from once I type in the recipient's address, based on past correspondence, like Newton did. But neither of these are even small issues.
In a more serious note, are you actually using eudora? If so I'd love to hear if there is a still-maintained version as well as the reasons sticking to it.
Eudora was recently donated to the Computer History Museum¹, so there's some faint hope of a revival, although it would more likely be the inferior Windows version.
¹ http://www.computerhistory.org/atchm/the-eudora-email-client...
Sure, they've - allegedly - made it run faster but they've also removed extension support, blocked any ways to hack in a calendar (which could previously be done by adding a permanent browser tab pointing to Google Calendar) and seriously hurt legibility with its new design by removing dividing lines, decreasing contrast across the board and giving all the icons a bright random colour.
A real shame as there are no real alternatives.
Last example of today: Filtering emails whether they have an attachment. A mail with attachments i was searching for was not included in the selection.
I use it with my personal email (hosted at Hover), and my work email, hosted on some IMAP server.
Works like a charm.
Overall quite happy with them.