How to show as busy across multiple Outlook or Microsoft 365 accounts

You have two or more Outlook or Microsoft 365 accounts — a work tenant and a personal outlook.com address, or two separate client tenants — and neither one knows the other exists. Book something in account A at 2pm and account B still offers 2pm to anyone checking your free/busy there, because Outlook's free/busy lookup stops at the tenant boundary: a calendar can only answer “is this person free” for people inside its own organisation, and the cross-tenant sharing Microsoft does support usually needs an admin on both sides to switch on. There are three ways to close the gap: add the other mailbox as a secondary calendar in your own Outlook client (only you see it), forward invites into one shared calendar (leaks details, one direction only), or mirror plain “Busy” blocks between every account automatically so each tenant's own calendar is accurate on its own. If you're juggling more than one Outlook tenant, or mixing Outlook with Google, mirroring is the only one of the three that actually stops the double-booking.

Why free/busy stops at the tenant boundary

Exchange and Microsoft 365 free/busy lookups work automatically inside a single organisation — everyone's calendar is in the same directory, so a scheduling assistant can check availability without asking permission. The moment you have two separate tenants, that stops. A personal outlook.com account and a work Microsoft 365 tenant are always two unrelated islands; there is no admin relationship to turn on between them at all. Two business tenants can set up an organisational relationship or cross-tenant sharing policy that exposes free/busy between them specifically, but in most tenants that is off by default, requires an admin on both sides to configure, and even when it exists it only covers the two tenants that agreed to it — not your third client, and not a Google account.

The practical result is that every Outlook account you hold is, by default, blind to what's booked in the others. Nobody misconfigured anything — this is how tenant isolation is supposed to work. It just also means nothing stops the double-booking unless you do something about it yourself.

Manual option: add the other mailbox in your Outlook client

If you have permission on the other account, Outlook desktop and web let you add it as an additional mailbox, or open its calendar directly if it has been shared with you. Once added, you see both calendars side by side when you're picking a time for yourself — genuinely useful, and free.

The catch is that this changes only what yousee. It does nothing to what the other tenant's free/busy service tells other people. Colleagues inside tenant A still see only tenant A's calendar when Outlook checks your availability for them, so they can still book straight over something that only exists in tenant B. This fixes your own view, not the underlying problem.

Manual options: forwarding invites and copying blocks by hand

A step further is setting a rule to forward or auto-accept invites from one account into another, so at least one calendar has a full picture. It works, but it is one-directional — the receiving calendar learns about the others, while the others still learn nothing — and it copies subject lines, attendees, and bodies across a tenant boundary, which is often exactly what a client contract's confidentiality clause exists to prevent.

The safer manual version is copying a plain “Busy” placeholder by hand into every other account whenever something lands on one of them. No detail leaves the source tenant. It is also the first thing that breaks: with two accounts it's tedious but survivable, with three or more it's a matter of time before a reschedule doesn't get copied and a slot silently goes stale in one of the calendars.

Mirroring busy-only blocks between accounts — including mixed Outlook and Google

The automated version of the placeholder trick is a busy-only mirror: connect every account you hold — as many Outlook/Microsoft 365 tenants as you have, plus any Google calendars — and a sync rule copies just a plain “Busy” block, with no title, no attendees, and no body, from each source calendar onto every other calendar you pick as a destination. Because the copy lands inside that tenant's own calendar, its own native free/busy lookup now correctly reports you as unavailable, which works even for colleagues who have no cross-tenant relationship with your other accounts at all. Reschedule or cancel the original event and the mirrored copies follow — nothing is left stranded the way a hand-copied block would be.

BusyBoard is the automated way to run this: it mirrors busy-only blocks between your own connected calendars whether the source and destination are both Outlook, both Google, or a mix of the two, and the source event is never modified — only a copy is written to each destination. If you'd rather share more than just “busy” between two calendars you own, titles or full details are available as an option per pair, but busy-only is the default and the one that leaks nothing.

Offering time without exposing which account it came from

Even with every account mirrored, how you offertime still matters. A booking page that only reads one of your Outlook accounts will happily offer a slot your other tenant already used. Send availability that merges every connected calendar into one answer, shown to the recipient in their own time zone — they see when you're free, never which employer, client, or tenant the busy time came from. That merged answer is the other half of solving this; mirroring keeps each calendar honest, and a merged link is what makes the honesty visible to the person trying to book you. See how to see all your calendars in one view for the general version of this across any mix of providers.

What's free and what's Pro

Connecting one calendar and sharing your availability from it is free — one connected calendar, one share link, a two-week booking window, syncing every ten minutes. Mirroring busy-only blocks between several connected accounts, whether that's two Outlook tenants or Outlook alongside Google, is a Pro feature (€9.90/$9.90 a month or €99/$99 a year, tax-inclusive), with a 14-day free trial that doesn't ask for a card. If you only ever need to look at your other calendar yourself, Outlook's own added-mailbox view and the free plan already cover that.