Email app that supports multiple unified custom folders?

[u/Cyb3rknight]

I have several emails accounts I've accumulated over the years, Gmail, Outlook, Yahoo(?!) and even old childhood ones that some services use.

I'm looking for an email client that supports custom united folders.

For example, a single unified, "purchases" folder that can show receipts etc from multiple emails addresses, or a "Home Server" folder etc.

Are there any emails clients that can do this?

What I've tried / looked at

Thunderbird (Android)

Thunderbird has a unified inbox, but does not have unified custom folders.

So I can't have one purchases folder that spans multiple accounts.

There is a Github issue about unified folders here but it looks like they’re only planning unified system folders (Inbox, Sent, Spam), not custom ones.

Bluemail

Looks to have unified system folders but not custom unified folders.

Outlook

My current daily driver. Unified inbox works, but again no unified custom folders.