Metre2 Mail - free, with Pro extras
Pin the emails you need to deal with and they stay above everything else until you're done - press P, free. With Mail Pro, tag mail in colour - Urgent, Waiting, Invoice, Site or your own - press C, and see every email in a category in one click.

Sheet 01PinnedCategories
Press P, click the pin on a row, or choose Pin to top from the right-click or More menu. It moves under PINNED at the top - and stays unread if it was.
Press P again, or choose Unpin. Undo is there if you change your mind.
Settings > Categories: Add starter set gives you Urgent, Waiting, Invoice, Personal and Site - or name your own, in eight colours.
Select an email and press C, or choose Categorise..., and tick as many as you like. Dots show in the list and pills in the reading pane.
Click a category under CATEGORIES on the left, or a pill on an email, to see every email in it - pinned ones first.
Help: Folders, pins and categories has every option in detail.
Free, with Pro extras. The parts marked Pro come with Mail Pro. Mail Pro is a one-off £9.99 upgrade inside the app, bought through the Microsoft Store - no subscription, no account. Try it free for 7 days.
Yes. Pin to top is free. Coloured categories are part of Mail Pro (a one-off £9.99, with a 7-day free trial).
Where your provider allows it, pins and categories are saved with the email on the server, so they survive a reinstall and another PC with Metre2 Mail sees them. Most phone apps don't show them. For POP3 and Microsoft 365 mode, pins are kept on this PC.
Yes - in Microsoft 365 mode they're your mailbox's own categories, both ways.
As many as you like, in eight colours. An email can have several.

Work and school mail through Microsoft Graph - even where IMAP is switched off.

PST and OST files folder by folder, .mbox and .eml too - duplicates skipped.

Summaries free; reply drafts and translations with Pro - nothing leaves your PC.

Three free rules, and one right-click to make one. Unlimited with Pro.

Blueprint paper in light, deep navy in dark - every email on clean paper.

Tracking pictures hidden, links cleaned, every email sandboxed, fakes flagged.
Metre2 Mail is free. Mail Pro is a one-off £9.99, with a free 7-day trial.