I came here to suggest something similar: in Thunderbird, it has this feature called "tags" where users can create their own tags and apply them to messages. Combine that with message filter rules that can apply tags based on matching a set of rules, you can automatically apply a number of tags to messages. Then, you can filter a mail folder by tags, as desired.

Instead of tags, you _could_ have rules file messages into separate folders, but I prefer having all the mail in one folder and using tags that can be applied in a boolean AND/OR method, so I can view "all messages tagged both [Important] and [To Do]" for example. I may have some messages that are important but have no action that are only tagged "Important", or messages that are actionable but not important which are only tagged "To Do" ... but, it's important for me to keep on top of those messages that are _both_ "Important" and "To Do" so I don't let them slip.

To be more explicit, this system is possible with a single free Gmail account -> https://support.google.com/a/users/answer/9282734?hl=en#zipp...