What happens to a Microsoft 365 mailbox when an employee leaves?

Someone hands in their notice, IT runs the offboarding checklist, and at some point a Microsoft 365 license gets freed up — because paying for a mailbox nobody reads feels wasteful. A month later, someone from accounting asks for an invoice confirmation that only ever existed in that mailbox. This is the moment many companies discover how Microsoft 365 (or Office 365, as half the internet still calls it) actually handles a former employee's email.

The 30-day countdown

Microsoft 365 doesn't keep mailboxes around out of goodwill. The moment you delete a user account (or leave a mailbox unlicensed), a countdown starts: the mailbox enters a soft-deleted state and is typically permanently deleted after 30 days. No warning email, no second chance. Whatever wasn't preserved somewhere else is gone — attachments, contracts, customer promises, all of it.

So what are your options when someone leaves? There are essentially four, and each has a catch.

Option 1: Keep paying for the license

The simplest option: just don't remove the license. The mailbox stays exactly as it was.

The catch is obvious — you're now paying a monthly license fee, forever, for an account nobody uses. Multiply that by everyone who leaves over the years and it becomes a quietly growing line item. It also leaves an active, sign-in-capable account lying around, which your security posture won't thank you for.

Option 2: Convert to a shared mailbox

The popular "free" trick: convert the mailbox to a shared mailbox before deleting the user, and the license is freed while the mail remains accessible.

It mostly works, with caveats that tend to surface later:

• The conversion must happen before the account is deleted — miss the window and it's too late.
• Shared mailboxes are only free below 50 GB. Above that, you need a license again.
• The mail is still fully editable and deletable by anyone with access. As a compliance record, it proves nothing.
• Ten leavers later, you have ten orphaned shared mailboxes that someone has to remember not to clean up.

Option 3: Export a PST file

Export the mailbox to a PST file, park it on a file share, delete the account. Feels like archiving — it's a copy, after all.

In practice, PST exports are snapshots that are hard to search, easy to lose, and trivially easy to modify. They cover exactly one moment in time, and their long-term fate is usually a folder called Old Employees that nobody has opened in years. We wrote a whole post on this: PST files are not an archive.

Option 4: Inactive mailboxes via retention policies

Microsoft's official answer: place the mailbox under a retention policy or litigation hold before deleting the user, and it becomes an inactive mailbox — preserved without a license, accessible via eDiscovery.

The requirements are where most small businesses fall off:

• It needs the right license tier at the time of deletion (typically Exchange Online Plan 2 or an E3/E5-class plan).
• The hold must be configured before the user is deleted. Most teams discover the feature afterwards.
• Getting mail back out means an admin running eDiscovery searches — not something a colleague does on a Tuesday to find one invoice.

The alternative: preserve mail while it arrives, not when someone leaves

An email archive flips the timing. Every message is captured and preserved automatically the moment it's sent or received — while the employee still works for you. Offboarding then becomes boring, in the best way: delete the account, free the license, done. The archive keeps the complete mailbox history, searchable in seconds, unchanged by anyone.

This is exactly what mailvault365 does for Microsoft 365: connect your tenant once, and every mailbox — including the ones of people who will resign three years from now — is archived continuously. When someone leaves, their archive simply stays, and "preserve the mailbox" disappears from your offboarding checklist permanently.

Wondering whether Microsoft's built-in tools already cover you? That's the next thing to check: Does Microsoft 365 archive email automatically?