I've been using Thunderbird on desktops and laptops, and K9 mail on Android, for years. Works fine. All you need is an IMAP server. Mine is hosted by Sonic and comes with my DSL service.

I really wish I could recommend K9 to anyone, but the search functionality is beyond broken. (I used to use it for FastMail because their own app loads oxymoronically slowly, but I just couldn't deal with the broken search functionality.) Have you found a way to work around this?

Unless you work for a company that disables POP/IMAP, for allegedly security reasons.

On Debian, you can just go to "Online Accounts" and add a Google Account, then your Google Mail will appear in Evolution Mail. Not sure what protocol is behind it, but it works when IMAP doesn't. Bonus: Evolution Mail comes with PGP support built in.

Is that better than Mail for Mac? My only need is reliable syncing and fast update, which Mac's mail is ok at, but needs variable amount of time for refresh, and if I need to verify my email, I mostly just open Gmail web.

I have never used a Mac for personal use, so I can't compare.

It does have per-account adjustable syncing periods, and you can click a big ol' button to refresh immediately.

Opera 12 for me solely for the mail client.

(They did spin off Opera Mail as a separate app, but I see no point in it, since it's just Opera 12 with the browser part disabled).

I recommend "The Bat!".

It's a bit clunky, but it's the best for serious people.

I've been using TheBat! around 2000-2003, back in dial-up days. It wasn't free back then (Opera wasn't either), and isn't free now.

It was/is great, but I didn't find a compelling reason for myself to shell some bucks over their way vs. using Opera Mail or Thunderbird. What are the killer features for you?

there are single(!) player games now that advertise that you can play them without internet connection, as a feature.

And sort by sender.

And search, in a way in which you can trust the search actually covered all your e-mails.

Actually search is the worst feature of Thunderbird in my opinion and I use Thunderbird for years and think it's great otherwise. Search constantly doesn't find something even if I use the keyword thay was in the email (verified using Inbox) or poorly sorts them (relevance algorithm is really weak) or displays result threads in inconsistent manner (many emails are duplicated etc.)