Has been using build in opera client for years
Now when opera moved to webkit and stripped it of important features I am switching browser and looking for mail client
Thunderbird disappointed me, zimbra seems very interesting and solid

One thing that bothers me though is the view and its options.
First I would say that something as important as how your mail clients windows look, separate panes and positioning of stuff might deserve some deeper level of preferences.
But since the default feels very natural and well made, its not as lacking as I expected it to be.

Anyway, one of the most important attribute I want to know about email when it comes in to my unified inbox, is to which one of my several accounts that mail belongs.
Some email accounts are important, others deserve only quick glance before deletion.
Seems you are working on it, because I can see greyed out account option for collumn, but when I somehow got it to show the it, by switching between conversation and message view,
it just showed gmail icon, not the account name, which was greatly disappointing.

Also it behaved strangely when I was switching between views and conversations and inbox and all mail, once it was selected other times not, next I had two columns, it seemed buggy.

Also, I have large monitor and I am used to having reading pane on the right, but preference for choosing columns by right clicking on the top toolbar are only possible with the reading pane on the bottom, I would really welcome some control over the columns even when panels on the right or possibility to have info about mails in multiple rows.
Though I can get used to reading at the bottom I guess... its less important than knowing to whom the mail is addressed without needing to go through them

Otherwise it really feels very nice