Zimbra is a unique and uncommon term that refers to a particular brand of email and collaboration software. However, there are a few viable synonyms that can be used interchangeably with the word. For instance, you could use the word "Outlook" as a suitable synonym since it's another well-known email client offering similar functionality. Alternatively, "Thunderbird" can be used as another synonym for Zimbra, as it's another email application that provides a wide array of features for effective collaboration. Finally, "Gmail" could also be considered a possible synonym since it's a popular web-based email service that allows users to easily communicate and collaborate with others.