When comparing Zimbra Web Client vs Squirrelmail, the Slant community recommends Squirrelmail for most people. In the question“What are the best self-hosted webmail clients?” Squirrelmail is ranked 8th while Zimbra Web Client is ranked 12th. The most important reason people chose Squirrelmail is:
Squirrelmail has been around for a long time.
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Pros
Pro Full-featured collaboration solution
If you are looking for an alternative to Gmail, look no further than Zimbra. While not a complete document solution it does have some document features as well.
Pro Very mature
Squirrelmail has been around for a long time.
Pro Has many available plugins
Can do things like change Linux user passwords, configure vacation "out of office" messages, configure procmail .forward files, etc.
Cons
Con Heavy and monolithic
Zimbra doesn't much work with its host platform as much as it brings its own copies of utilities. If you want something small and "plays well with others" integratable...this is not it. Too heavy to run on a RaspberryPi if you're looking for something for just a few users.
Con Linux only
There is no installator for Windows
Con Very old
Squirrelmail has been around for a loooooong time!