I've been using Thunderbird for several years, I got really annoyed with the Windows mail app and Outlook for countless problems. Thunderbird has always been very simple and works great and clean.
Recently - I think version 60 or so, broke minimize to tray, very annoying. But someone fixed it and came out with a new version. It's a feature that must be included in Thunderbird from the get go.
Another great feature is being able to copy a profile folder over to Thunderbird from another machine to another and I have all my emails bang up to date.