Microsoft 365 shared mailboxes: licences and archiving

An address such as invoices@ or support@ belongs to a team rather than one employee. A shared mailbox lets several people handle the same correspondence through their own accounts. Questions usually arise when the mailbox grows. Is it free? Does an archive increase its capacity? Can you convert a departing employee’s mailbox and immediately remove the licence?

A basic shared mailbox can hold up to 50 GB without its own licence. People accessing it need a licensed Exchange Online mailbox. Additional capacity, archiving and advanced features have separate requirements. A setting being available in the admin centre is not proof of entitlement. Microsoft explains the baseline in its shared mailbox overview.

Separate three different requirements

Handling current correspondence, storing older messages and preserving content are different tasks. Your organisation may need all three, but each requires an appropriate feature and entitlement.

The primary mailbox is the team’s working space. An online archive is additional mail storage. Retention and holds determine how content is preserved or removed. An archive alone does not decide which records the business must keep or for how long. Moving messages to the archive does not increase the primary mailbox’s limit either.

Before choosing a licence, ask whether you need a larger primary mailbox, space for older messages, protection against deletion, or a combination. “We want to archive everything” is too vague to turn into an order.

Decision guide

This table follows the Exchange Online limits and their notes. The notes matter: they distinguish primary storage, archive storage and feature entitlements.

Why the admin centre may not prove entitlement

Forum discussions describe mailboxes where an archive can be enabled without the expected licence. Some replies interpret that as free capacity. For your environment, record the service, assigned products and specific provision covering its use.

Do not compare your tenant with another administrator’s screenshot alone. Mailbox history, older configurations and subscriptions may differ. If documentation and service behaviour appear inconsistent, ask support a precise question: which product covers this particular archive? The answer should reference a rule, rather than merely confirm that the setting works.

Convert a departing employee’s mailbox

Conversion is useful when colleagues need to continue handling the correspondence. It is not an automatic solution for preserving all of a former employee’s work data.

1. Check the primary mailbox size, archive status and active preservation rules.
2. Decide who will use the converted mailbox and for what purpose.
3. Convert it and verify the resulting mailbox type.
4. Assign access to named colleagues and block direct sign-in to the original account.
5. Test incoming mail, access and sending through their own accounts.
6. Only then assess licence removal against capacity and feature requirements.

Microsoft provides a user-to-shared mailbox conversion procedure. In a hybrid environment, also check where the mailbox type and related attributes are managed.

Access and sending are different permissions

Distinguish opening the mailbox, sending as the shared address and sending on behalf of it. Recipients see the difference. A message from invoices@ is different from a colleague’s message sent “on behalf of invoices@”.

Prepare two tests: a colleague opens an incoming message, then sends a reply in the required way. Folder access alone does not verify the entire workflow. Agree where the team will find sent replies and how it will know a customer has already received an answer.

Having everyone sign in with one shared password makes this harder to manage. It becomes difficult to identify who handled content, and every departure creates another shared-access issue. Use personal accounts and delegated permissions.

Hypothetical example: a 72 GB sales mailbox

A hypothetical sales mailbox holds 72 GB in its primary area. The team uses it daily and needs to find recent enquiries quickly. Converting it to a shared mailbox does not make it eligible for the unlicensed 50 GB allowance.

The administrator must make two separate decisions: the required primary capacity and the way older mail will be handled. If archiving is chosen, the team needs to know which messages move and how to find them later. Licensing and configuration must cover both requirements.

Preservation of contractual correspondence also belongs in the decision. The sales manager defines the content needed, the responsible person confirms preservation rules, and IT checks implementation. Random bulk moves of messages should not replace the organisation’s rules.

Prevent the mailbox from filling up

Monitor its size, growth and owner. Alert the owner while a change can still be planned, rather than after customer mail is rejected.

• Distinguish large attachments from ongoing growth.
• Check for unnecessary automated copies arriving in the mailbox.
• Verify that archiving actually runs.
• Discuss every planned preservation change with the content owner.
• Repeat incoming and outgoing mail tests after a change.

Do not promise savings simply because a licence is removed. If the seat remains part of a paid commitment, changing its assignment may not reduce the current invoice.

Prepare the information for a licensing review

Record the exact mailbox type, primary and archive sizes, assigned products, active preservation features and roles with access. These details support a much more reliable decision than the general question “are shared mailboxes free?”