Metre2 Mail - free
A meeting invitation shows as a card at the top of the email: what it is, when - in your own time - where, who organised it and who else is asked, with their answers so far. Accept, Tentative or Decline in one click, with a note if you're running late, and your answer goes to the organiser the standard way, so their calendar updates. Online meetings get a Join button. Free in Metre2 Mail.

Sheet 01InvitationAccept / Tentative / Decline
Invitations from work calendars, shared calendars and booking sites show a card above the email: the meeting, the day and time in your own time zone, the place, how often it repeats, the organiser and who else is invited - with who has already said yes.
Press Accept, Tentative or Decline. Metre2 Mail emails your answer to the organiser from the account the invitation came to, the standard way, so their calendar shows it.
Add a note... sends a line with your answer - "I'll be ten minutes late" - which the organiser sees alongside it.
The card says what you answered. Change answer shows the buttons again. If the organiser moves the meeting, the card says it was updated since you answered.
Online meetings get Join online. With Mail Pro, Add to Metre2 Calendar puts the meeting in your calendar as it came.
Help: Reading email has every option in detail.
Free in Metre2 Mail. No time limit, no ads, no account, and your mail never passes through our servers. Mail Pro is there when you want every account in one inbox, categories, AI drafts and unlimited rules.
Yes. Reading and answering invitations is free for everyone. Adding the meeting to Metre2 Calendar is part of Mail Pro.
Yes. Your answer goes to the organiser as a standard calendar reply - the same kind work calendars and shared calendars send - so their calendar marks you as accepted, tentative or declined.
Metre2 Mail doesn't keep a calendar itself. With Mail Pro, Add to Metre2 Calendar puts the meeting in Metre2 Calendar, our free calendar app for Windows.
Your own time. A meeting set in another time zone is shown at the matching time where you are.
A cancelled meeting shows its card crossed through and marked Cancelled, with no buttons to press.

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.

Pin to the top with P, free. Colour-code with C in Mail Pro.

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

Tracking pictures hidden, links cleaned, every email sandboxed, fakes flagged.

Click the bell as you send, and get a nudge if nobody replies. Mail Pro.

Every newsletter in one list: unsubscribe and tidy your inbox in one go. Mail Pro.
Metre2 Mail is free. Mail Pro is a one-off £9.99, with a free 7-day trial.