Thunderbird!
I realise that enterprise users will have a different take on this, but as an avid user of Outlook since it first appeared, and despite experimenting with the likes of Scalix etc, I never found a real competitor.
However, as MS continued to remove useful features, meaning using even more COM addins to get them back, and with the effort that was put into Thunderbird, I decided to give Thunderbird a try. It has progressed to the point now where, with just one extension, it does everything I used to use Outlook for and more. As a result I abandoned Outlook some 2 years ago, and haven't really missed it.
I use TB with both google workspace, POP3 and Exchange accounts without problems.
MS Mail etc were worse than the very earlist mail clients I ever used (and I've been around since before the internet existed!) Nothing I see here convinces me they are significantly improved.
Just saying...