ClearOS, formerly known as ClarkConnect, sits in an interesting part of the Linux landscape. It is not a general-purpose desktop distro in the same sense as Ubuntu or Fedora rather, it is commonly deployed as a small-business gateway, firewall, directory, mail, and server platform with a web-based administration model. That matters a great deal when choosing an email client. On ClearOS, the “best” mail application is usually not the prettiest one, but the one that fits the package base, works reliably on the desktop environment available, and does not make life difficult for an administrator who wants something supportable, predictable, and easy to maintain.

In practice, ClearOS installations are often used as headless systems, or as light desktop environments on a management workstation. When a GUI is present, the most common choices tend to be lightweight or mainstream desktop stacks, with a bias towards stability over novelty. ClearOS also leans strongly towards RPM-based packaging in the broader Red Hat family tradition, which immediately narrows the field. For this reason, the most suitable clients are the ones that ship as RPMs and have sensible integration with standard Linux desktop components such as GNOME, KDE, or basic GTK/Qt stacks.

For ClearOS, I would focus on five clients from your list: Thunderbird, Betterbird, Evolution, Mailspring, and the desktop apps from Proton Mail and Tuta Mail. Of those, the Proton and Tuta clients are especially worth including because they are practical, modern options for people who want end-to-end encrypted mail workflows on a Linux desktop. On ClearOS, however, their suitability depends heavily on whether the workstation has the right runtime support and whether the packaging format fits the installed base cleanly.

Below is a distro-specific comparison, with an emphasis on what actually makes sense on ClearOS rather than on Linux in general.

Now to the practical part: why these particular clients make sense on ClearOS, and why the others are less compelling.

Thunderbird is the safest recommendation by a wide margin. On ClearOS, you want software that behaves predictably, survives long maintenance cycles, and does not demand unnecessary system changes. Thunderbird has long been the default answer for that sort of requirement. It supports IMAP, SMTP, calendaring add-ons, address books, and a broad range of extensions. Importantly, it is also available as an RPM, which keeps it aligned with the way ClearOS administrators tend to think about software lifecycle management. If the desktop environment is modest, Thunderbird still works well enough on a small office machine without becoming invasive.

Evolution deserves serious consideration on ClearOS when the machine is a GNOME-based workstation or at least has a good GTK stack installed. Evolution is not merely an email client it is a groupware front end. That makes it particularly relevant in a small business setting where calendar invitations, contacts, and possibly Exchange-style back ends matter. On ClearOS, that is useful because many deployments are used by owners or admins who manage shared calendars, service addresses, and internal coordination. The RPM package means it integrates cleanly with the distro’s package habits.

Proton Mail is especially appealing if the user already relies on Proton’s encrypted ecosystem. For a ClearOS desktop, the RPM package is the main reason it earns a place in this shortlist. It avoids the awkwardness of forcing a Flatpak or AppImage workflow onto a distro that is otherwise oriented around native packages. Proton Mail is not the most flexible traditional mail client for mixed corporate environments, but for privacy-conscious users it is one of the best-supported choices.

Tuta Mail is similarly privacy-led, but its packaging options are a little less convenient here. Flatpak can work nicely on a desktop-oriented ClearOS installation, though in a server-centric environment there is often some hesitation around sandboxed software stacks unless the workstation policy is already familiar with Flatpak. AppImage is also convenient, though less integrated. If the user’s priority is encrypted mail and calendar usage with minimal system dependency, Tuta is reasonable, but it is not my first pick for a typical ClearOS desktop.

Mailspring is neat, modern, and simple enough for light use, but on ClearOS it is more of a secondary option. Its RPM package is helpful, yet it does not offer the same confidence level as Thunderbird for long-term admin-friendly deployment. It is best suited to users who want a slick interface and basic email handling rather than a mature “do everything” desktop mail suite.

Betterbird is worth mentioning because it improves and refines Thunderbird behaviour for some users, especially those who are sensitive to interface details or Thunderbird quirks. However, because it is distributed as a tar.xz archive rather than an RPM, it is less convenient on ClearOS. That does not make it a bad choice it simply makes it a slightly more hands-on one. On a workstation managed by a capable administrator, it is perfectly viable.

Some of the other clients from your list are technically excellent, but less suitable for this specific distro.

KMail / Kontact can be a strong option on KDE-heavy systems, but ClearOS is not especially associated with KDE desktops. If the workstation happens to be KDE-based, it becomes more interesting otherwise, it feels like a less natural fit than Thunderbird or Evolution.

Claws Mail is light and efficient, and on a modest machine it can be attractive. Still, its look-and-feel tends to appeal to users who already know they want a minimalist mail tool. On ClearOS, where the usual need is reliability and straightforward administration, Claws Mail is competent but not especially compelling unless hardware is limited.

Sylpheed is in a similar category: stable, light, and technically sound, but somewhat old-school in presentation. For a ClearOS workstation used by someone who wants a traditional mailbox UI and little else, it can work well. In a modern business environment, though, Thunderbird or Evolution tends to be a better fit.

The TUI clients, namely aerc, NeoMutt, and Alpine, are excellent tools for experienced administrators. In fact, on a headless ClearOS box or a text-mode management session, they may be the most “native” tools of all. But the moment the machine is acting as a standard desktop workstation, their use becomes a matter of personal preference rather than a broad recommendation. If your ClearOS deployment is mostly server-side and you access mail through terminal sessions, NeoMutt or aerc can be fantastic. For a GUI-centric user base, they are niche.

That leaves the two best practical recommendations for the typical ClearOS desktop: Thunderbird and Evolution, with Proton Mail as a strong third if encrypted mail is a key requirement.

The reasoning is simple:

• Thunderbird gives the best balance of maturity, package availability, and broad mail protocol support.
• Evolution is excellent where calendars, contacts, and office-style groupware matter, especially on GNOME-based systems.
• Proton Mail is the cleanest privacy-first option with native RPM support, which matters a great deal on ClearOS.

How to install and configure the best options on ClearOS is where things become more practical. The following examples assume a ClearOS system with the RPM package manager available in the usual Red Hat-style manner. If your deployment uses a web-managed software catalogue or a curated repository set, the package names may still match but repository availability can vary.

First, Thunderbird.

Thunderbird is generally the easiest to deploy and the least controversial choice. On a ClearOS workstation, it is usually best installed via RPM if available in your configured repositories. If not, the upstream tarball can be used, but the RPM route is preferable because it keeps updates cleaner.

sudo dnf install thunderbird

If your ClearOS version uses yum rather than dnf, the command is similar:

sudo yum install thunderbird

After launching Thunderbird, the basic configuration is straightforward:

• Add your email address and password.
• Choose IMAP rather than POP3 unless you have a very specific reason not to.
• Set incoming mail server, outgoing SMTP server, and port details according to your provider.
• Enable OAuth2 where supported, especially for modern hosted mail systems.
• Set a default folder subscription policy if you are handling shared mailboxes.

If you are connecting to a business mailbox, IMAP over TLS and SMTP submission over TLS are the right defaults. On ClearOS, where network and security policy matter, that keeps behaviour sane and avoids unnecessary local mail storage.

Second, Evolution.

Evolution is a very strong choice for office users on a ClearOS desktop. It is especially useful if the user wants mail, calendar, and contacts in one place. The installation is again best done through the native package manager, assuming the RPM is available from the enabled repositories.

sudo dnf install evolution

Then open Evolution and use the account wizard to set up mail. A sensible workflow is:

• Select IMAP for the mail account.
• Provide the incoming and outgoing server details.
• Turn on TLS/SSL wherever the provider supports it.
• Configure calendar integration if your account supports CalDAV, Microsoft 365, or Exchange-style access.
• Import contacts if you are migrating from another client.

Evolution is particularly useful when the ClearOS workstation is used by someone who is not especially technical but still needs a polished business suite. It feels more integrated than Thunderbird for calendaring and contact management, which can be a real advantage in a small office.

Third, Proton Mail.

Proton Mail is the best option for privacy-focused users who want a desktop client and prefer a native RPM package. On ClearOS, that native packaging is a real asset. The exact package name may vary slightly depending on how Proton distributes the RPM at the time, but the process usually follows the same pattern: download the RPM, install it, then sign in with your Proton account.

sudo dnf install ./protonmail-desktop.rpm

If the package is already in a repository or local path, the installation can also be handled directly by RPM or DNF. After installation, launch the app and authenticate with your Proton account. A few practical points matter here:

• Expect the app to focus on Proton services rather than acting as a universal mail client.
• Use it when the security model of Proton is the reason you selected it in the first place.
• Keep the system updated, because desktop apps that integrate with encrypted services should always run on a clean, current base.

For Tuta Mail, the installation story is slightly different because it is offered as Flatpak or AppImage. If you are on a ClearOS workstation with Flatpak support already established, it can be installed that way. AppImage is simpler to try quickly, but it is less integrated from a system administration standpoint. That makes Tuta convenient, but not quite as neat as Thunderbird, Evolution, or Proton Mail on this distro.

In terms of overall ranking for ClearOS, my practical order would be:

1. Thunderbird
2. Evolution
3. Proton Mail
4. Tuta Mail
5. Mailspring

If you are running a ClearOS box primarily as a server or gateway and only occasionally accessing mail locally, then the terminal clients can still be worth a look. In that scenario, NeoMutt is probably the most established choice, aerc the most modern-feeling terminal experience, and Alpine the most traditional. Still, for most users on ClearOS, a GUI client will be the better day-to-day fit.

To round things off, there are a few mail services that work particularly well with the clients above and suit the kind of user who runs ClearOS.

• StartMail — A good privacy-conscious option if you want a straightforward webmail and IMAP-friendly service. It fits well with Thunderbird and Evolution, and it appeals to people who want a cleaner privacy posture without overcomplicating the desktop setup.
• Fastmail — One of the best choices for professionals. It is well-behaved with IMAP, CalDAV, and CardDAV, which makes it excellent with Evolution and Thunderbird on a ClearOS workstation.
• Proton Mail — A strong recommendation if encryption and privacy are the priority. It pairs naturally with the Proton desktop client, and the native RPM availability makes it especially suitable here.
• Tuta Mail — Another privacy-first provider that works nicely for users who want an encrypted ecosystem with a desktop app. It is a good match for people who are comfortable with Flatpak or AppImage on ClearOS.

For a ClearOS deployment, the sensible answer is not to chase the newest mail client, but to choose one that respects the distro’s RPM-first, business-oriented nature. Thunderbird remains the most dependable all-rounder, Evolution is the strongest office suite-style option, and Proton Mail is the best specialised privacy choice. If you keep those three in mind, you will cover most ClearOS desktop scenarios properly, without creating unnecessary maintenance work for yourself later on.