RoundCube Webmail is an open source webmailing program, under the GPL 3 license. The license of a program tells if it is/ if you can call it ›open source‹. OSS (open source software) can be used, copied and distributed (i.e. provided via your private or public web server for whomever it wanted), read (that is why you call the sources of the program »open«) and changed as you like!
Default setting for Roundcube is to use text-only emails, without html; the web surface sucks little data from the web, not a whole bloat of images, code and scripts like at a commercial provider as GMail.
OSS usually comes with little bloat, no DRM (digital rights management, a bad thing that executes arbitrary code on your computer), and no unnecessary features. It is regarded more secure since a possible backdoor would be visible in the sources and since independent security experts are allowed to check the logiciel (fr; software) and develop & provide security fixes on their own. This is an issue, since the fix of a bug can or often still can be bogus! With proprietary software even if you knew a security issue, you could not fix it - not being able to see the source code, nor were you allowed to do so.
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