If Microsoft acquires Yahoo, you can be sure Zimbra will be one of the Yahoo properties that Microsoft jettisons, neglects, or otherwise shutters.
There’s no way Microsoft, presuming it becomes the proud owner of Yahoo, will allow Zimbra to compete with its Outlook franchise, despite all the great reviews, including this one, that Zimbra has deservedly earned.
Fortunately, there are Zimbra versions that are licensed under the Yahoo Public License, and Microsoft isn’t going to be able to make them just disappear. The non-YPL’d materials might be made into un-software, and I think it’s likely that sales of new support Agreements would end. Use and distribution of the existing software, under the terms set forth in the YPL, can continue in perpetuity, and there’s nothing Microsoft can do about it.