Nah, Mailbird is way behind Opera Mail. Though it's prettier and more contemporary I couldn't even find out how to quickly filter messages by the address, which I love in Opera, moreover with its small font so that you can see lots of topics and quickly orient in sent and received messages. Maybe because after three days of usage of Mailbird it came out that the trial is over! Only three days? That's not the client-oriented approach.
Yandex Mail is OK and its keyboard text tips are useful despite they are appreciably slower than for example the Mac ones. Still, it's a web-client so you cannot see income / sent messages for several mailboxes at once. But their servers really helped. Yandex.Connect service is great. We moved our mail there and now Opera Mail works with much fewer issues.

@ericar Couldn't have found it. Did you mean MailBird?

Hello to Opera Mail from the future...
Guys, I am one of many lovers of the Opera Mail software (and of the old Opera browser, not a new one) and really regret stoppage of its development. I haven't found any email client with such a convenient grouping of messages by the subject; neither desktop nor webmail. Are there any? Or other developers cannot embed it in their software? Not to mention the Opera's great spam filter.
The issue with the email client is freezes on several PCs in the office simultaneously. They happen too often and last several minutes. It's not the Ethernet issue, and the webmail clients also work well. The problem is none employee can try any other desktop client, 'coz they are so inconvinient and clumsy -- it's almost impossible to work using them.