I'm looking for a FOSS email client for desktop. (Windows and MacOS, for now.)

Using Airmail or Mailbird as references, I'm looking for a client with some of these features:

• A clean, minimalist design
• A left-side toolbar as a narrow band, icons only.
• Bonus points if it can schedule an email to send later.

I'm familiar with Thunderbird and there's a lot to like there, but design-wise, it's pretty cluttered, IMHO.

Does anyone know of a FOSS desktop email client that looks clean? -OR- Is there a way to change Thunderbird's layout so it looks/functions more like Airmail or Mailbird?

Thank you!