After asking this question it looked like K9 was the answer for me.

Now I see that there is no Windows version of K9, so here's the same question again, but for Windows & Android, the same client for both preferably, please.

I use Thunderbird as my Windows desktop email client. There is no Thunderbird for Android.

I actually quite like the Gmail client, but do not want Google reading all of my mail.

I need something which :

  • will handle multiple accounts (at least a dozen), and keep them separate
  • has good search capabilities (message body too, not just header) partial sender address, wildcard searches, date range searches, all of these combinable
  • a bunch of plugins would be nice
  • I especially want GPG support
  • and I don't want it scanning my emails & using me as "the product"
  • has IMAP support
  • allows me to schedule how often it checks for new mail (not once an hour, like Gmail)
    • supports common mailbox formats, so that I could switch to a new client & import
  • tags are nice
  • mark as "in need of reply" & remind me

I don't need a calendar or fancy address book, but won't object. Low resource usage would be nice. Stable, with good support & an active user community.

Gratis (and add-free) would be great, but I am willing to pay, if there is a compelling reason. I don't need open source, but don't object to it.

  • 2
    Afraid no such thing exists. There might be one client for both systems, but secure and for both systems I don't know. What's so wrong about using K-9 for Android, and a different one for Windows? // PS: if you are "willing to pay", the gratis tag contradicts that (as it says this is a requirement).
    – Izzy
    Commented Jun 17, 2018 at 20:09
  • 1
    I removed the gratis tag. I will settle for secure-ish. The main thing is that I don't want Google looking at my email. Preferably no large companies. I have many email addresses, for many purposes; most are on my own servers, and I am going to get rid of any Gmail, etc Multiple address/in-boxes, plus GPG would do it. Default is Thunderbird and K9, but it would be nice to use a single app on both platforms
    – Mawg
    Commented Jun 18, 2018 at 6:34

2 Answers 2


Protonmail is a very good option.

  • Welcome balon! Please expand on your answer, and explain how it meets the question's requirements. Thanks! Commented Dec 5, 2019 at 9:46
  • I know that it is highly secure, which is good. Does it also run on Windows?
    – Mawg
    Commented Dec 5, 2019 at 11:35

Looking at https://www.guidingtech.com/k9-mail-app-alternatives-android/ I find the curiously named Tutanota.

Weird name but awesome email client and one of the few names in the market that can give ProtonMail a run for its money. Another open-source email client with a presence on both Android and iOS, Tutanota also uses end-to-end encryption but misses out on OpenPGP.

That allows them to encrypt the subject line (unlike ProtonMail), but to read the encrypted message, the recipient will have to click on a link and visit Tutanota website and enter a password.

One additional feature is 2FA. which secures your mobile and desktop clients. Yes, Tutanota also has native clients for Windows and macOS. Based out of Germany, which is governed by GDPR, Tutanota offers more space in the free plan, and their paid plans begin at €12/year.

Tutanota’s UI is better than K-9 Mail but less attractive than ProtonMail. There is support for swipes which is configurable, but no snooze or self-destructing emails yet

I am not quite certain about this (because it uses its own servers, although all email is encrypted), but at least it offers clients for both Android & Windows, so I will post this in case it helps someone else.

More possibilities - all Windows & Android, free; I need to investigate these:

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