I want to jump into this because recently I had to test both for some specific project I was working.
Let’s resume it like this. I had the need to create an advanced folder/filtering/tagging system for some log emails. I’m talking about 1000+ emails per day which gets nicely organized into color folders and filtered out based on specific trigger words.
So I started to test Gmail (because storage is free) and a local Zimbra server (not free). The filtering in Gmail is not bad, and the search is very powerful. But then I started to hit into some limitations. So I started to test the rules in Zimbra. Not only I was able to replace the same Gmail rules (while no labels, it has tags which is similar) but I could create far more complex rules. And the search in Zimbra is absolutely phenomenal as well, believe it or not, even while Gmail is from Google, Zimbra’s build in search (I think it’s using Elastic search) is superb.
I liked how emails are incoming in real time with Gmail but then again, you can also do that with Zimbra. Zimbra has just more features, and if you are talking about business collaboration I think it’s the better choice, in particular with the sharing folder and email features which means no more sending emails internally in the same organization.
I went with Zimbra for this particular project. And while Gmail rocks for private use, I would have second thoughts about business use. You may be violating some privacy policy by storing them on Gmail. Some specific type of business can’t store them on Gmail because of their scanning content issues and advertising. Actually they were just sued in the US again for this. While this may be less important for individuals, you may have to think about that. Some emails in particular in specific industries or governments have sensitive information which don’t allow them to be used with third party services that scan and read the content, no matter how automated it is.