I’m a UX designer and I think this product is great. There are a few tweaks I would suggest.
The placement of search, I wish this could be to the left and over the inbox items or contacts. That is a more natural way to work.
Missing delete icon right on the inbox so I can delete with a click. I used this all the time on Microsoft Mail.
The calendar appointment dialog is too busy. Increasing the spacing between fields would help.
The dark theme could do with dimming some of the borders on input boxes. Try the dark theme then create a calendar entry. The gridlines in the calendar are too bright, they should be more subtle.
Otherwise thank you. I have been looking for an alternative to Microsoft Outlook/Mail for a long time.