Yes there are lots of open source webmail interface in market, which are lot good.
One of them which I use is and recommend is group-office open-source from https://www.group-office.com/ Also Horde Groupware webmail edition is great with support for mobile sync for calandar /contact/task/notes
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I Am Still using GroupOffice ..and its too fast compare to gmail for me .
Yes there are lots of open source webmail interface in market, which are lot good.
One of them which I use is and recommend is group-office open-source from https://www.group-office.com/ Also Horde Groupware webmail edition is great with support for mobile sync for calandar /contact/task/notes