Our Team uses Office 365 which includes Outlook for email. I have used Outlook forever. I also have a Gmail account that I rarely use. I have multiple emial address and all, except the Gmail, forwards to my Outlook.

I moved away from Outlook after Office 2000, never like the ribbon.  I've used Thunderbird for a couple decades now it seems and I'm very happy with it.  Not easy to put the calendar on a phone though, so I still use an online one for that.

K9 Mail is best for the mobile phone.

I use Outlook. Always have. I will continue to do so until I retire. 

Outlook has all the features and functions I need. For me, it is simple and easy to use. 

We use Outlook.  We are happy with it.  Like with everything in life one size does not fit all.  A

Fred - in my opinion, any method of communication between two parties to be effective should be agreed upon upfront by those parties.  Agent states I use this email method - client says yeah(or nay) - proceed.  Maybe, I am not fully understanding your question.  

I use MacMail and LOVE it! I send videos out to my agents each week and embedding them in MacMail is UBER easy. I love the filing system and the overzealous spam filtration system (MOST days)!! LOL

I am an Outlook guy because that is what I use. Real techie here!

I use outlook, a lot of functionality that I cant get elsewhere.

I tried Thunderbird for a short time however it was sloww and bogged down a lot.

I use Outlook and have for over 20 years. 

I'm Old School and use Top Producer 8I.  I know how to use it and am comfortable with it.  It's affordable and efficient.  Their Mass Mail-out option is fantastic.  Their Customer Service will walk you through any question about their product.

I use Outlook, as it makes our lives easier with folders and organization.

I use several. There's a reason for that.
For examle MailChimp is a common resource for those who are unaware that MailChimp is recognized by most ISP as the tool many use to paste the world with their solicitations. The ISP automaticly flag MailChimp as undeserable. Not a good resource for pasting those not established in ones directory.
When the need to paste the world with my solicitation I want to use a disposable asset. My responses to their acceptance of my offer will come from another source that has a common identifier. 
The important stuff comes from a domain email provided by GoDaddy. Owning such actually provides unlimited email addresses but can result in the entire domain being flagged if not protected.
1. One provider for pasting. (disposable)
2. Another for nurturing. Usually CRM. (WiseAgent, Sugar)
3. Another for the real work attached to a branded website URL. Because of back end requirments this is typicly a agent/broker type CRM, WiseAgent.

It is noteworthy that #2 includes 'inbound' marketing strateges that do not rely on email but are short duration 'pasting' operations. This resource represents more than 50% of all "haven't met' leads.

I use Thunderbird which has been ok for several years, also use gmail and verizon which is actually aol. I am seriously considering starting up an account with fastmail. https://www.fastmail.com/features/

You don't mention G Mail in there. Lots of add ons but you can make the whole email thing just too complicated.

I use Outlook & I was just surprised to find out that my version (2016) does not keep records over 2 years. How's that supposed to work for the IRS records?

I have Outlook and it is the best for me in RE business. I'm used to it and it is convenient and organized.
I have several Gmail accounts for those whose spam filters won't let my domain mail past, and for 'junk'. Also for dealing with folks who primarily use it.
I have ProtonMail and frankly haven't used it much, but if I need it it is there. 

I use Thunderbird, used Outlook in the past but when my mail file got corrupted and could NOT be restored and I lost everything, that was it for me, lost trust in it.  I know many use Outlook with no issues but I wasn't as lucky. 🤬