Thunderbird works great for me across different desktop operating systems. It doesn't have the best UI but it is all IMAP and it handles multiple inboxes/calend

Thunderbird works great for me across different desktop operating systems. It doesn't have the best UI but it is all IMAP and it handles multiple inboxes/calendars (custom domains and an old gmail account) just fine. You can also tweak the settings for each inbox accordingly. I use the default mail app on iPhone.