Zoho Mail's free tier doesn't include IMAP (for new accounts since about 3 years ago), so you wouldn't be able to use Thunderbird, FairEmail, K-9 Mail, or other third-party apps. Offline access on the free tier is restricted to Zoho Mail's desktop and mobile apps. Zoho's Mail Lite plan ($1/user/month) does include IMAP access, and you'll need to do some calculations to see whether Purelymail or Zoho Mail is cheaper.

https://purelymail.com/advancedpricing

https://www.zoho.com/mail/zohomail-pricing.html

It’s only 1 domain I think and they stripped out things like ActiveSync and IMAP a couple years ago IIRC. I use the $1/user/month plan and it’s decent. I don’t use calendars or contacts, so I’m not sure how that stacks up, but the email stuff is good.

The biggest annoyance is that someone used mx, mx2, mx3 for their MX instead of mx1, mx2, mx3, so the dns record indentation doesn’t line up. Lmao.

I've tried Zoho mail and the "problem" is that it's so much more than mail. It's a huge suite of online apps, really. That means if you purely need mail, it'll take a bit of time to find the setting you need.

(It's been three years since I tried it, and looking at my notes, I couldn't get Zoho to sync contacts for me, and somehow didn't get calendar notifications.)

I have a custom domain email address on Zoho and suffer constantly from delivery issues. Sometimes my emails go into spam filters, sometimes they go into a black hole. It’s a huge PITA and I am tempted to either move providers (maybe it’s Zoho’s fault) or just stop using my custom domain (maybe it’s not Zoho’s fault).

Well, I'm no expert and I'm not in any position to say anything negative about Zoho Mail... but the free tier you mention seems to allow for one domain. So it likely won't work if you have, say, 5 active domains you want to receive and send email.

I’ve used Zoho for a while and you can add on domains. It’s pretty cool and the online mail is a pretty decent UI.