Gmail AlternativesOnly Open Source apps categorised as Calendar Apps

The best open source Calendar alternative to Gmail is Proton Mail. If that doesn't suit you, our users have ranked more than 100 alternatives to Gmail and six of them are open source Calendar Apps so hopefully you can find a suitable replacement. Other interesting open source Calendar App alternatives to Gmail are Tuta Mail, Posteo, Infomaniak kMail and Kolab Now.

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Gmail alternatives are mainly Calendar Apps, but if you're looking for WebMail Providers or Email Clients you can filter on that. Other popular filters include Free, Open Source, Android + Free, EU, Android + Open Source, Android and Linux. You can also filter by region, for example EU-based alternatives if you prefer software developed in the European Union. These are just examples - use the filter bar below to find more specific alternatives to Gmail.

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Gmail alternatives page was last updated May 2, 2026

Proton Mail
1086 likes
Proton Mail provides secure email services with end-to-end encryption, ensuring data protection. Swiss-based and outside US jurisdiction, it offers a free version to champion privacy as a human right. Fully accessible on any device.
104 Proton Mail alternatives
Cost / License
Freemium
Open Source (GPL-3.0)
Application types
WebMail Provider
Email Client
Origin
Switzerland
Platforms
Mac
Windows
Linux
Online
Android
iPhone
iPad
Snapcraft
Flathub
Tor
+9
Is Proton Mail a good alternative to Gmail?

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34comments
Proton Mail vs Gmail Comments

Top Positive Comment
John Fastman
Dec 20, 2016
19
Protonmail is free, open-source, encrypted (more private). Lacks: calendar & good contacts feature, but these are in the pipeline. Protonmail cannot by design data-mine your software for it's own profit; Gmail makes it's money by violating your privacy that way.
Shaz Shah
Positive commentMay 21, 2025
1
Proton Mail is my primary email and I have been moving away from GMail. In fact I do pay for it.

Proton Mail has been reliable and their support has been great. The UI is different and pleasant, and they have apps that cater for different platforms (Windows, macOS, Android, etc).
Charlignon
CommentMar 3, 2025
1
Protonmail is pretty great for all reasons listed. BUT it has two major drawbacks that you need to be aware of :

Account may be deleted due to inactivity (1 year - free plan) -SMTP (for apps like K9 mails or Thunderbird) requires additional setup, and is not available on mobile devices These were deal breakers for my use cases

3 of 34 comments
Proton Mail is Freemium and Open SourceGmail is Free and Proprietary
Proton Mail is Privacy focusedGmail is not according to our users
Tuta Mail
563 likes
Offers comprehensive encryption for email, calendar, and contacts with zero personal info required. Built on open source, install via Play Store, F-Droid, or APK, with Tor and VPN support. Both free and paid plans; utilizes quantum-resistant encryption ensuring privacy-focused, anonymous usage.
109 Tuta Mail alternatives
Cost / License
Freemium
Open Source (GPL-3.0)
Application types
WebMail Provider
Email Client
Origin
Germany
EU
Platforms
Mac
Windows
Linux
Online
Android
iPhone
iPad
Flathub
F-Droid
Android Tablet
Thunderbird
+1
Is Tuta Mail a good alternative to Gmail?

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13comments
Tuta Mail vs Gmail Comments

Top Positive Comment
justinterested
Jan 6, 2019
8
Tutanota is a great email provider with very good encryption (it uses end-to-end encryption like Signal – i.e. they cannot read your emails/gife them to the government, Gmail can!) and they don't sell any data. It is also very easy to use, it has a very nice app, a desktop client and a web client. There is really no reason why you wouldn't want to use it :)
Top Negative Comment
Guest
Jul 14, 2019
25
Does not support POP/IMAP, you will not be able to use it with Thunderbird or another email client.
Review by a new / low-activity user.
alttosgt
Positive commentApr 1, 2025
0
Tuta Mail is NOT self hosted despite claim on AlternativeTo
3 of 13 comments
Tuta Mail is Freemium and Open SourceGmail is Free and Proprietary
Tuta Mail is Privacy focused and LightweightGmail is not according to our users
Posteo
89 likes
Web- and IMAP-based email service. 4GB cost 1€/month. Information is hosted on encrypted hard drives. Electricity from renewable resources is used.
79 Posteo alternatives
Cost / License
Paid
Open Source
Application types
WebMail Provider
Email Client
Origin
Germany
EU
Platforms
Online
+9
Is Posteo a good alternative to Gmail?

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5comments
Posteo vs Gmail Comments

Top Positive Comment
RemovedUser
Apr 6, 2019
1
The most secure and private email on the market. As such, there is not free version. This is a good thing, it means they are not making money with your information.
Review by a new / low-activity user.
Top Negative Comment
Guest
Sep 12, 2024
0
Lack of labels - only folders. Poor and broken search function on web client (seems to scramble email details in results?) Unable to send through external addresses. Does not group multiple separate emails from same sender. No multiple star colours
Review by a new / low-activity user.
RemovedUser
Positive commentApr 13, 2018
3
Very comprehensive privacy, affordable price and sustainable practice. Also provide interface for notes, calendars and contacts sync.
Review by a new / low-activity user.
3 of 5 comments
Posteo is Paid and Open SourceGmail is Free and Proprietary
Posteo is Privacy focused and Security-focusedGmail is not according to our users
Ad
Proton Mail
Proton Mail provides secure email services with end-to-end encryption, ensuring data protection. Swiss-based and outside US jurisdiction, it offers a free version to champion privacy as a human right. Fully accessible on any devi...

https://proton.me
Learn More
Infomaniak kMail
48 likes
Privacy-focused Swiss email system provides 20 GB free storage, supports custom domains and unlimited storage, syncs contacts and calendars, is ad-free, and is part of a secure productivity suite used by over a million users across Europe on all devices.
50 Infomaniak kMail alternatives
Cost / License
Freemium
Open Source (GPL-3.0)
Application types
WebMail Provider
Email Client
Origin
Switzerland
Platforms
Online
Android
iPhone
Android Tablet
iPad
F-Droid
+8
Is Infomaniak kMail a good alternative to Gmail?

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3comments
Infomaniak kMail vs Gmail Comments

Guest
Positive commentOct 10, 2017
1
You have all the functions you need in workspace, like Gmail. Fully customizable, high security. This product is professional. I use it since 2 year, and I don't want to return to Gmail. Only one point : you need a domain name. But it's more original like a Gmail.com address...
Review by a new / low-activity user.
ImSebi
Positive commentFeb 16, 2022
0
Much better than Gmail. And now you don't need a domain anymore, it's free for everyone.
hb9vaf
Positive commentSep 24, 2025
0
I very good alternative for mail, contact & agenda
Review by a new / low-activity user.
Infomaniak kMail is Freemium and Open SourceGmail is Free and Proprietary
Infomaniak kMail is Privacy focusedGmail is not according to our users
Kolab Now
43 likes
Kolab Now provides a suite of intuitive apps in a user friendly environment to make your day to day online comms easier, smarter and most importantly; secure.
76 Kolab Now alternatives
Cost / License
Paid
Open Source
Application types
WebMail Provider
Mail Server
Origin
Switzerland
Platforms
Online
Software as a Service (SaaS)
DAVx5
Is Kolab Now a good alternative to Gmail?
Horde
30 likes
Enterprise-level, browser-based suite for IMAP email, calendar, contacts, tasks, notes, wiki, and file management, offering syncing, shared and unlimited resources, multilingual support, customizable dashboard, and integration with mobile and external clients.
Cost / License
Free
Open Source
Application types
WebMail Provider
Email Client
Platforms
Online
Self-Hosted
Is Horde a good alternative to Gmail?

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2comments
Horde vs Gmail Comments

Guest
Negative commentNov 12, 2015
9
Horde is a mail client, not provider
Review by a new / low-activity user.
Guest
Positive commentSep 25, 2017
0
Horde is included in CPanel accounts as well as Roundcube and Squirrelmail.
Review by a new / low-activity user.
Horde is Free and Open SourceGmail is Free and Proprietary
Horde is Privacy focused and LightweightGmail is not according to our users

6 of 6 Gmail alternatives

Gmail AlternativesOnly Open Source apps categorised as Calendar Apps

Alternatives list

Proton Mail

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Proton Mail vs Gmail Comments

Tuta Mail

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Tuta Mail vs Gmail Comments

Posteo

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Posteo vs Gmail Comments

Infomaniak kMail

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Kolab Now

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Horde

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Horde vs Gmail Comments