Zentyal Server, formerly known as eBox Platform, sits in an interesting spot in the Linux world: it is not a general-purpose desktop distribution, but a server-oriented platform built around Ubuntu LTS foundations and managed through a web interface. In practice, that changes the email-client conversation quite a bit. On a Zentyal box you are usually dealing with a headless system, a handful of administrators, and a workload centred on mail services, Active Directory-style integration, DNS, firewalling, and groupware features rather than everyday desktop use. So the “best” mail clients are not simply the prettiest ones they are the ones that fit neatly into Ubuntu/Debian packaging, work well over remote administration, and behave sensibly in a server-adjacent environment.

Because Zentyal tracks Ubuntu LTS under the hood, Deb packages are the most natural choice, with Flatpak sometimes useful if you do have a desktop session on an admin workstation attached to the server. Zentyal itself is commonly managed remotely, and its target users tend to be SMB admins, sysadmins, and IT generalists rather than end users. That means compatibility, stability, and predictable updates matter more than flashy integration features. GNOME-based desktops such as Ubuntu Desktop, GNOME on Debian, and some Xfce or MATE setups are also common in the wider ecosystem, but on the server itself you will often be logging in via SSH or accessing the web admin console rather than running a full GUI locally.

With that in mind, the most suitable clients here are the ones that are mature, well-packaged for Ubuntu/Debian, and can be used for either admin mailbox access or testing mail flow. For a Zentyal environment, the strongest choices are usually Thunderbird, Proton Mail, Tuta Mail, Evolution, and, for more advanced users, Betterbird. Among those, Thunderbird is the safe default, Evolution is a strong GNOME-integrated option, Betterbird is excellent if you want Thunderbird compatibility with extra polish, and Proton/Tuta are relevant if your organisation uses privacy-focused hosted mail rather than a traditional IMAP/SMTP setup.

Below is a practical comparison tailored to Zentyal Server’s Ubuntu-based nature and typical deployment style.

Now, looking at the list in more practical terms for Zentyal Server:

Thunderbird is the most sensible recommendation. Zentyal administrators often need to verify IMAP authentication, SMTP submission, folder permissions, spam handling, and inbox delivery from a client that behaves like a normal end-user mail application. Thunderbird does this very well. It is widely documented, stable on Ubuntu LTS, and available as a .deb package if you are working from a Debian/Ubuntu desktop. It also supports add-ons, which can be handy when you are validating mail flows or integrating with calendars and address books in a small office environment. For a Zentyal deployment, this matters because you can use Thunderbird both for administrative checks and as a daily driver on the same Ubuntu-based workstation.

Evolution is the best fit if your admin workstation is GNOME-based, which is common around Ubuntu. It tends to integrate neatly with desktop calendars, contacts, and enterprise-style accounts, so it is a useful companion when Zentyal is acting as a small business mail platform. Its package availability as a .deb is a plus on this distro family. Evolution feels especially appropriate in office environments where the server is just one part of a broader Ubuntu desktop deployment.

Betterbird is worth choosing if you like Thunderbird’s model but want a slightly improved user experience. It uses tar.xz packaging rather than native .deb from the project itself, so on Zentyal it is a little less integrated than Thunderbird. Still, for a technically minded administrator, Betterbird can be a strong option when you want Thunderbird compatibility with some practical enhancements. It is a “good fit” rather than the “default fit”.

Proton Mail is relevant if the organisation uses Proton as its hosted mail provider in parallel with Zentyal, or if you need a secure desktop client on a remote admin machine. The Debian package support makes it usable on Ubuntu-based systems, but it is not the first choice for directly validating Zentyal’s own IMAP/SMTP services because it is designed around Proton’s ecosystem.

Tuta Mail is similar in that respect. It is perfectly respectable on a desktop admin station and it does offer Flatpak, which can be convenient on Ubuntu-based desktops. However, Zentyal mail environments are usually more traditional in how they expose mailboxes, and Tuta is more about accessing Tuta’s service than interacting with a local server stack. That makes it a niche choice for this distro.

Geary is tidy and simple, but in a Zentyal context it often feels too minimal for serious administrative work. If your task is just reading mail from a local IMAP account, it is fine. If you need to troubleshoot quotas, multiple identities, message routing, or enterprise mail behaviour, Thunderbird or Evolution is generally better.

KMail / Kontact would make more sense on a KDE-heavy workstation than on a typical Zentyal management environment, so it is not among the first choices here. Mailspring is polished, but the packaging options listed are less compelling for a Zentyal admin context than the established GNOME/Thunderbird options. Claws Mail and Sylpheed are lightweight and efficient, but they are more old-school and less convenient for modern enterprise mail workflows. The TUI clients such as aerc, NeoMutt, and Alpine are excellent for SSH-heavy administrators, but they are more specialist than most Zentyal deployments need they are ideal if you live in terminal sessions, not if you want a straightforward GUI for checking server behaviour.

For Zentyal Server specifically, I would narrow the shortlist to three practical winners:

• Thunderbird for the default, best-supported choice.
• Evolution for GNOME-centric admin desktops and calendar/contact integration.
• Betterbird for users who want Thunderbird compatibility with a few sensible refinements.

Here is how I would approach installation and setup on a Zentyal-based Ubuntu/Debian environment.

1) Thunderbird

Thunderbird is the easiest recommendation because it matches the distro family cleanly and supports traditional mail protocols well. On a Zentyal-connected workstation, I would typically install it from the Ubuntu/Debian repository or from Mozilla’s package source, depending on the system policy. Once installed, it should be configured against your Zentyal mail domain using IMAP and SMTP submission rather than POP3, unless you have a very specific reason to do otherwise.

Typical setup steps:

sudo apt update
sudo apt install thunderbird

Then in Thunderbird, add the account manually if auto-discovery does not pick up the correct values. For a Zentyal server, your administrator details usually look along these lines:

IMAP server: mail.example.lan or mail.example.com
IMAP port: 993
Security: SSL/TLS
SMTP server: mail.example.lan or mail.example.com
SMTP port: 587
Security: STARTTLS
Authentication: normal password
Username: full email address, for example user@example.com

Use the Zentyal certificate chain if you are running your own internal CA, and make sure the certificate presented by the mail service matches the host name you are using. In a small business setup, that detail is often where client-side warnings begin.

2) Evolution

Evolution is a strong option if the admin workstation runs GNOME, which is common on Ubuntu Desktop. It handles mail, calendars, and contacts in a fairly enterprise-friendly way, and that makes it well suited to organisations using Zentyal as an all-in-one server for users who expect groupware-style behaviour.

Installation via APT on Ubuntu-based systems is straightforward:

sudo apt update
sudo apt install evolution

After launching Evolution, create a new mail account and point it at your Zentyal IMAP and SMTP services. In the account wizard, select the mail protocol manually if necessary and verify the encryption settings. If your Zentyal deployment uses a local LDAP directory or other central user store, Evolution can work well alongside that, though the client itself is still configured per user account rather than through server-side magic.

On GNOME desktops, I would favour Evolution where the user needs a combined outlook-like experience: mail plus calendar plus contacts. It is neat for office administrators who want one application rather than several.

3) Betterbird

Betterbird is slightly more manual because the project distributes tar.xz archives rather than a neat .deb package in the listing here. That said, it is still very usable on Zentyal’s Ubuntu base, particularly if you are comfortable with unpacking an application bundle in a user-local location.

General installation approach:

tar -xf betterbird-.tar.xz
cd betterbird
./betterbird

For regular use, many administrators place it in a stable directory under the user profile or /opt, then create a desktop launcher if needed. Configuration is almost identical to Thunderbird, because the underlying account model is closely related. If your team already knows Thunderbird, Betterbird will feel immediately familiar.

On Zentyal, Betterbird makes sense when you want Thunderbird behaviour but prefer a slightly more responsive or polished variant for daily mail work. It is less compelling if you want easy system-wide deployment managed entirely through apt.

From a technical standpoint, Zentyal’s strengths also define the mail-client choice. Since it is commonly used as a unified small-business server, mail is often tied to DNS, SSL certificates, internal naming, and user directories. That means your mail client should not just “connect” it should cope well with custom certificates, local hostnames, and standard IMAP/SMTP authentication. Thunderbird and Evolution are especially good at that. Proton Mail and Tuta Mail are fine desktop clients, but they are much less useful if your actual aim is to verify the behaviour of a Zentyal mail service, because they are built around their own secure ecosystems rather than a generic business mail stack.

In short, for Zentyal Server, I would treat Thunderbird as the default choice, Evolution as the best GNOME-integrated alternative, and Betterbird as the specialist pick for admins who want Thunderbird compatibility with a bit more comfort. If the workstation is also handling privacy-first hosted mail outside the Zentyal domain, then Proton Mail or Tuta Mail can be added on top, but they are not the first tools I’d reach for when setting up or testing the server’s own mail services.

For compatible email services, I would particularly recommend the following for a Zentyal-based office where some users want hosted mail and others use the local server:

• Proton Mail — a strong privacy-first option, especially if some staff want encrypted hosted mail alongside the Zentyal server.
• Tuta Mail — good for users who value simplicity and privacy, and who do not need deep integration with the local Zentyal stack.
• Fastmail — excellent for business users who want dependable IMAP/SMTP access and clean desktop-client compatibility.
• Mailfence — a sensible pick for organisations that want privacy-conscious hosted mail with standard protocol support.

Of these, Fastmail is often the easiest pairing with desktop clients such as Thunderbird and Evolution because it speaks standard mail protocols very cleanly. Proton Mail and Tuta Mail are the better fit when privacy and encrypted hosted mail are the priority. Mailfence is also a decent middle ground for business use where standards compliance matters more than ecosystem lock-in.