Network Security Toolkit (NST) is a rather specialised Linux distribution, and that matters a great deal when choosing an email client. It is not a general-purpose desktop in the usual sense NST is primarily built for network troubleshooting, security analysis, packet inspection, and web-based administration. In practice, that means the “best” mail manager is not necessarily the one with the most features, but the one that behaves well on a security-focused system, does not add unnecessary complexity, and fits the way NST is typically used.

Most NST deployments I see are used by technical users, often from a live environment or on a system with a conservative package base. Depending on the release and how NST is installed, the available desktop may be lighter and more traditional than a full-blown consumer Linux desktop. That usually points towards clients that are stable, fairly lightweight, and easy to integrate with the system’s package manager. On that basis, the most suitable options from your list are Thunderbird, Betterbird, Evolution, and, for those who want a hosted privacy-first workflow, Tuta Mail and Proton Mail. I would also keep Geary in the conversation as a simpler alternative, but on NST it is usually less compelling than the big three desktop clients.

Below I have tailored the comparison to NST specifically, rather than giving a generic Linux overview.

For NST specifically, I would rank the options like this:

1. Thunderbird
2. Betterbird
3. Evolution
4. Geary
5. Tuta Mail and Proton Mail as service-tied options rather than general desktop mail clients

Thunderbird comes out on top for NST because it offers the widest package coverage and is forgiving on mixed or somewhat constrained desktop environments. NST users often work with a mix of tools, possibly in sessions that are not meant to be polished consumer desktops. Thunderbird is familiar, robust, and handles IMAP, POP3, SMTP, filtering, calendars, address books, and extensions without forcing you into one ecosystem.

Betterbird is the sensible choice if you like Thunderbird but want a more polished day-to-day experience. It is effectively Thunderbird with practical improvements, which can matter on a technical workstation where you want mail to “just get on with it”. The main caveat is packaging: with only a tar.xz available, you are relying more on manual placement and updates rather than tight integration with NST’s package manager. Still, on a security toolkit, that is not necessarily a downside if you want to keep application management under your own control.

Evolution is worth considering on NST if you are not just reading mail but also managing calendars and contacts in a more enterprise-style fashion. It is especially attractive in GNOME-based desktops, and Flatpak support means you can often install it even where the base repository is not ideal. However, it is heavier and more opinionated than Thunderbird, so for a toolkit-style distro it may feel like more than you need unless you genuinely rely on groupware features.

Geary is the neat, minimal option. It is visually clean and works well for IMAP accounts, but it is not the best choice for a security toolkit where the user may need advanced mail rules, multiple identities, integration with logs or certificates, or broader protocol support. On NST, I would only choose Geary if the goal is a simple mailbox client with very little overhead.

As for Tuta Mail and Proton Mail, they deserve mention because they are privacy-centric and frequently requested by users who handle sensitive communications. On NST, they are suitable only when your mail is already hosted with them. Tuta is friendlier from a packaging perspective because it provides Flatpak and AppImage, which are often easier to run on varied systems. Proton’s desktop app is available as .deb and .rpm, which is fine if NST’s base is compatible with one of those formats otherwise it is less convenient than Thunderbird.

It is also worth pointing out what I would not prioritise on NST. Mailspring, KMail/Kontact, and some of the lighter legacy clients can work, but they are not as convincing here. Mailspring is serviceable but not especially compelling unless you like its interface. KMail/Kontact makes most sense in a KDE-heavy environment, which is less common for NST. Claws Mail is excellent for advanced users, but its configuration model and plugin-centric style can feel a little old-school unless you specifically enjoy that approach. TUI clients such as aerc, NeoMutt, and Alpine are technically excellent, but they suit console-first administrators more than most NST users, who often already have enough terminal work in their day.

From a desktop-environment point of view, NST users commonly end up in lighter environments such as Xfce, MATE, or Openbox-like setups rather than heavyweight GNOME or KDE sessions. That favours clients that start quickly, do not need extensive background services, and integrate cleanly with basic notification and authentication mechanisms. Thunderbird and Betterbird fit that description well. Evolution fits if you happen to be using a GNOME-flavoured NST setup or want calendar integration. Geary is visually at home in GNOME, but in a toolkit distribution it is more of a convenience client than a workhorse.

The package manager question matters too. NST is not the kind of distro where I would assume every modern packaging format is equally convenient in day-to-day maintenance. If the system is package-manager constrained, Thunderbird’s tarball or a native package is a strong advantage. Betterbird’s tar.xz is similarly portable. Flatpak is acceptable for Evolution and Geary, especially when the base system is minimal, but it does add a layer of runtime management. Proton’s .deb/.rpm packages are only ideal when NST is already comfortable with one of those ecosystems otherwise they can be awkward. Tuta’s Flatpak/AppImage pair is very flexible, which is helpful on a toolkit distro where you may not want to disturb the base system.

Below are the 3 best choices for NST, followed by practical installation and configuration guidance.

1) Thunderbird

Why I recommend it on NST: it is the most universally suitable, easiest to support, and least likely to surprise you. For an administrator or analyst, that reliability is valuable. It also supports a wide range of account types and security settings, which matters if you are dealing with multiple mail services, aliases, or PGP workflows.

Typical installation approach on a toolkit-style system is the tarball, because it avoids packaging mismatches. If you do have a package that matches your NST base, use that instead.

Example installation flow for the tarball:

cd ~/Downloads
tar -xf thunderbird-.tar.bz2
sudo mv thunderbird /opt/
sudo ln -s /opt/thunderbird/thunderbird /usr/local/bin/thunderbird

First-run configuration should focus on security and practical mail handling:

• Add the account manually rather than relying on aggressive auto-detection.
• Prefer IMAP over POP3, especially if NST is used intermittently or as a live environment.
• Enable OAuth2 where the provider supports it.
• Review attachment handling and message display settings, particularly if you inspect untrusted mail on the same machine.
• If you use PGP, install and test the relevant encryption toolchain separately before depending on it.

Common useful settings include keeping the message cache modest, disabling remote content by default, and using a dedicated profile for sensitive work.

2) Betterbird

Why I recommend it on NST: it is a strong choice if you like Thunderbird’s ecosystem but want a slightly sharper user experience. On a technical distribution, that can translate into less friction without sacrificing familiar behaviour. It is also a good choice if you prefer to keep the application self-contained.

Example installation flow for the tar.xz archive:

cd ~/Downloads
tar -xf Betterbird-.tar.xz
sudo mv betterbird /opt/
sudo ln -s /opt/betterbird/betterbird /usr/local/bin/betterbird

Configuration advice is almost the same as Thunderbird, but with a slightly more opinionated approach:

• Create a separate profile for operational mail versus personal mail.
• Turn off any features you do not need, especially if NST is used in a lab or incident-response context.
• Use IMAP with server-side folders so your mailbox remains accessible if the toolkit is booted from external media.
• Test sending and receiving with your provider’s SMTP settings before relying on the client for real work.

Betterbird is especially attractive when you want a familiar mail client without adopting a heavier desktop suite.

3) Evolution

Why I recommend it on NST: it is the best pick if mail is only part of the job and you also need calendar and contact integration. In a more operational setting, especially if you are managing team schedules or exchange-style accounts, Evolution can be genuinely useful. Its Flatpak option is helpful where NST’s native packages are limited.

Example Flatpak installation:

flatpak install flathub org.gnome.Evolution
flatpak run org.gnome.Evolution

Configuration guidance for NST:

• Use it when you need calendars alongside email, not just mail alone.
• Check whether your desktop session supports portal integration properly, especially for authentication dialogs.
• Keep the account list tidy Evolution can become busy if you add multiple identities at once.
• Use server-side search and folder organisation where possible, rather than relying on large local caches on a toolkit system.

In a GNOME-like NST setup, Evolution feels natural. In a lighter desktop, it can still work, but you will notice its weight more.

If you want a simpler fallback, Geary is worth a look. It is easier to live with than many think, but it is still best regarded as a straightforward IMAP reader rather than a full administrative mail station. For NST, I would choose it only when minimalism is more important than breadth of features.

Now, on to mail services that pair well with these clients on NST. The most sensible options are:

• Proton Mail — Strong privacy posture, excellent if you want an encrypted ecosystem and are happy to use a dedicated mail service. This is a natural fit with Proton Mail desktop or, for a more general client, Thunderbird with the right account settings.
• Tuta Mail — Very good if you want a privacy-first service that keeps the desktop story simple. It pairs nicely with Tuta’s own client, especially on a distro where you may prefer Flatpak or AppImage delivery.
• Fastmail — Excellent for reliability, IMAP/SMTP compatibility, and straightforward integration with Thunderbird, Betterbird, or Evolution. This is arguably one of the cleanest choices for NST if you want a professional mail platform without friction.
• Mailfence — A sensible privacy-aware alternative with broad client compatibility, including standard desktop clients. It is a good option when you want secure mail but prefer to avoid a single-vendor desktop app.

I would recommend Fastmail most strongly for NST users who want dependable standards-based mail with minimal fuss. For privacy-focused workflows, Proton Mail and Tuta Mail are the obvious choices, with Proton being the more common fit for mixed desktop usage and Tuta being particularly tidy if you intend to use its own client. Mailfence sits neatly in the middle for people who want a secure service without locking themselves too deeply into one desktop application.

In short, if NST is your daily security workstation, choose Thunderbird first, Betterbird if you prefer a slightly refined Thunderbird-like experience, and Evolution if calendars and contacts matter as much as email. If your mail account is already with a privacy provider, then Tuta Mail and Proton Mail are perfectly sensible, but they are service choices as much as client choices. For NST, simplicity, portability, and low maintenance usually win the day.