RISC OS Open is an unusual target from a modern mail-client point of view, and that is precisely why the choice matters. This is not a mainstream Linux desktop where you can assume a full GNOME stack, a large amount of RAM, or a package manager with every application under the sun. If you are running RISC OS Open in a Linux-hosted environment, or you are using it alongside Linux on ARM hardware where the wider desktop is still expected to do the heavy lifting, the practical answer is to favour clients that are light, standards-compliant, and realistic about their dependency footprint.

That means looking first at what is actually sensible for this environment: clients that are well-maintained, IMAP-friendly, tolerant of modest hardware, and available in package formats that fit the surrounding Linux system you are likely to be using. In real terms, that usually means deb on Debian-based systems, or flatpak where the desktop environment is a bit more mixed. For ARM boards and older machines, memory use and startup time matter far more than polish. A large, heavy client may be lovely on a big Fedora workstation, but it becomes a nuisance when resources are tight.

For RISC OS Open users, the best fit is usually a client that can handle standard mail protocols cleanly, sync reliably, and avoid dependency sprawl. In that light, the strongest candidates from your list are Thunderbird, Tuta Mail, Proton Mail, and, depending on the desktop stack, Evolution. A more lightweight desktop alternative such as Betterbird is also worth considering where it is available in a suitable package form, though note that its distribution packaging is more limited. Of these, Thunderbird is the safest all-round recommendation Proton and Tuta are attractive if you are already committed to their ecosystems, but there are compatibility caveats worth addressing.

Below is a practical comparison tailored to RISC OS Open’s likely usage pattern and the surrounding Linux environment.

Now, in RISC OS Open terms, the most suitable options break down quite clearly.

1) Thunderbird

Thunderbird is the most sensible default. It is the safest recommendation because it has broad package availability, excellent IMAP and SMTP support, good support for multiple accounts, and a familiar interface. For users running a conventional Linux desktop alongside RISC OS Open, Thunderbird is the least troublesome route. It is also the easiest of the mainstream clients to support over time because it is mature, well documented, and widely used.

On a typical ARM desktop or modest machine, Thunderbird is not tiny, but it is manageable. If you are using a Debian-based environment underneath, the deb package is usually the straightforward choice. If you prefer containerised apps and your desktop stack already supports it cleanly, the flatpak can reduce dependency friction.

2) Claws Mail

Claws Mail is a very strong choice where performance matters. It is fast, traditional, and comparatively light, which makes it appealing for systems where you do not want a heavyweight mail suite competing for RAM. It suits technical users who want straightforward mail handling rather than a highly integrated personal information manager.

For RISC OS Open users, this is often the best “serious but lean” alternative to Thunderbird. It is especially good if the machine is not new, if the Linux host is resource-conscious, or if you simply want a mail client that gets out of the way.

3) Sylpheed

Sylpheed is another lightweight, no-nonsense client that fits the ethos of a constrained or specialist environment rather well. It is simpler than Thunderbird, which is both its strength and its limitation. If your requirement is largely IMAP mail with a clean interface and minimal resource use, Sylpheed makes a lot of sense.

Where Sylpheed falls behind Thunderbird is in ecosystem richness and polish. Still, for a pragmatic user on RISC OS Open, that may be a perfectly reasonable trade-off.

4) aerc

aerc is the best TUI option if you are comfortable in a terminal. This is particularly relevant for RISC OS Open users who tend to work in lightweight desktop environments, remote sessions, or minimal Linux layers. aerc is efficient, fast, and very usable once configured, though it is not for everyone.

If you like keyboard-driven workflows and want to keep memory use very low, aerc is a serious contender. It is also a good fit for technically confident users who are used to managing mail without a large graphical suite.

5) Proton Mail or Tuta Mail, but only when the package format matches the Linux side of your setup

Proton Mail and Tuta Mail are both worth considering if privacy is a top priority. That said, they are not the universal answer for RISC OS Open, because compatibility depends more heavily on the Linux environment you are actually running. Proton provides deb and rpm, which makes it a realistic option on Debian/Ubuntu-style or Fedora-like systems. Tuta offers AppImage and flatpak, which can be convenient if the desktop has good Flatpak support.

For a RISC OS Open user, these are best seen as service-centric clients rather than general-purpose mail workhorses. They are excellent if you already use those providers and want local desktop access, but they are not the first choice if your priority is lightweight, broadly compatible desktop mail.

By contrast, I would not lead with clients such as Mailspring, KMail, or Evolution unless your environment already leans that way. Mailspring is polished but not especially lean. KMail is excellent in a Plasma world but brings KDE dependencies you may not want. Evolution is powerful, yet it feels more natural on GNOME and similar desktops than on a stripped-back setup.

Geary, Balsa, and Betterbird are all workable in the right circumstances, but none of them displace Thunderbird or Claws Mail as the more practical choices for this platform. Geary is too minimal for many technical users. Balsa is respectable but less commonly adopted. Betterbird is attractive, but its limited packaging means it is less convenient in heterogeneous deployments.

How to install and configure the best choices

Thunderbird

On a Debian-based Linux layer, Thunderbird is usually the easiest starting point. If you are using a package manager on the underlying Linux system, install it there rather than trying to force something exotic. On a desktop with Flatpak support, that is also a clean option.

Typical Debian-style installation:

sudo apt update
sudo apt install thunderbird

After installation, launch Thunderbird and choose “Add account”. Enter your name, email address, and password. If your provider supports automatic configuration, Thunderbird will usually discover the correct IMAP and SMTP settings. If not, switch to manual configuration and set:

• Incoming server: IMAP
• Outgoing server: SMTP
• Encryption: preferably STARTTLS or SSL/TLS
• Authentication: normal password or app password, depending on the provider

For IMAP-heavy setups, Thunderbird is particularly suitable because it handles folders, multiple identities, and address books without making a fuss. If you use two-factor authentication on your mail service, generate an app password where required.

Claws Mail

Claws Mail is usually the best lightweight desktop option. Install it from your distribution’s packages if available:

sudo apt update
sudo apt install claws-mail

At first start, the account wizard will ask for your display name, email address, and server details. In most cases you will want:

• Protocol: IMAP
• Incoming server port: 993 for IMAPS, or 143 with STARTTLS
• Outgoing server port: 587 with STARTTLS, or 465 with SSL/TLS if your provider requires it
• Authentication: normal password or app password

Claws Mail works best when you keep the setup simple. Use server-side folders and avoid overcomplicating filters unless you need them. For a low-resource system, that discipline pays off.

aerc

aerc is the best option if you prefer working in a terminal and want to keep the desktop overhead low. Installation on a supported Linux layer is usually package-manager driven:

sudo apt update
sudo apt install aerc

Configuration is more manual than Thunderbird. The usual approach is to create a configuration file under your home directory and define IMAP/SMTP account settings there. A minimal account setup commonly includes:

• IMAP server and port
• SMTP server and port
• username
• password storage method, preferably a secure helper if available
• mailboxes to synchronise

For example, your configuration should reference the provider’s IMAP and SMTP endpoints and use TLS. If you are using Proton or Tuta, check their official documentation carefully, because desktop integration and access methods can differ from ordinary IMAP accounts.

The main thing with aerc is that it rewards a tidy, methodical setup. Once configured, it is quick, elegant, and ideal for machines where every megabyte counts.

What I would choose in practice

If you want the most balanced answer for RISC OS Open, choose Thunderbird. If you want something lighter and more technical, choose Claws Mail. If you are very performance-conscious or live in the terminal, choose aerc. If your main concern is privacy and you already use those ecosystems, then Proton Mail and Tuta Mail deserve attention, provided your underlying Linux package support aligns.

In short, for this distro and its likely environment, the winning formula is simplicity, low overhead, and standards-based mail. That is exactly where Thunderbird, Claws Mail, and aerc tend to excel.

As for compatible mail services, the most sensible options here are Proton Mail, Tuta Mail, Fastmail, and Mailfence. I recommend them because they are privacy-conscious, work well with modern authentication, and are generally dependable for IMAP/SMTP or desktop-client use. Fastmail is particularly strong if you want an efficient, standards-friendly service with excellent reliability. Mailfence is also a solid choice if you want a secure service with proper desktop mail support. Proton Mail and Tuta Mail remain the best-known privacy-first services, though, as always, it is worth checking which client integration route best fits your exact Linux setup.