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Shared Mailboxes Done Right

When to use a shared mailbox, when not to, licensing limits, and the common misconfigurations that cause problems months later.

Last reviewed August 2026

Overview

Shared mailboxes are a handy Microsoft 365 feature but are often misused. Used correctly, they give a team a single inbox (info@, support@, accounts@) at no extra licence cost. Used incorrectly, they become licence dodges, break sent-item visibility, and cause compliance issues at offboarding.

When to Use One

  • Team inbox: info@, sales@, support@, accounts@
  • Departing staff mailbox converted to shared for ongoing access
  • Role-based inbox (rostering, dispatch, on-call)
  • Legal hold / retention beyond the user lifecycle

When Not To

  • As a licence saver for an active staff member (breaches the Microsoft licensing terms)
  • Where multiple people need to send privately (use delegation, not a shared mailbox)
  • Above 50 GB of mail (needs a full mailbox with licence)
  • When MFA / Conditional Access needs to be distinct per user
  • For high-volume customer support (use a ticketing system)

Licensing and Limits

  • Shared mailboxes up to 50 GB: no licence required
  • Above 50 GB: requires an Exchange Online Plan 2 licence
  • Do not sign in as a shared mailbox (sign in as yourself, add the shared mailbox)
  • Shared mailbox cannot sign in directly. Sign-in blocked by default, keep it that way
  • Auto-mapping: attaches in Outlook automatically when Full Access is granted

Setup Best Practices

Grant Full Access AND Send As / Send On Behalf

Full Access only means staff can read. To respond as the team, Send As or Send on Behalf is required.

Save sent items in the shared mailbox

Enable "Save copies of messages sent as this mailbox in the Sent Items folder" via PowerShell or Exchange admin.

Disable sign-in on the shared account

Block sign-in in Entra ID. Remove the password-based MFA. Mailbox is accessed via delegation only.

Apply retention policies

Shared mailboxes often hold long-term customer correspondence. Set retention explicitly.

Audit who has access quarterly

Departed staff often retain access. Review and remove.

Consider a Teams channel email instead

For some teams, a Teams channel inbox works better than a shared mailbox for daily triage.

Automatic Replies and Signatures

  • Shared mailbox auto-replies set via admin, not the user
  • Signature must be set per user sending from the shared mailbox (Outlook does not auto-apply a mailbox-level signature natively)
  • Consider Exchange transport rules or a signature management tool for consistent signatures
  • Test replies before relying on them (common misconfiguration)

Common Mistakes

Using shared mailboxes to avoid user licences

Microsoft licensing terms prohibit this. Audit risk and functional limits.

Sign-in not blocked

Shared mailbox credentials become an orphan account. Block sign-in.

Sent items going to the individual user

Team cannot see the reply thread. Enable shared sent items.

No retention policy

Mailbox grows indefinitely, breaches the 50 GB limit, or loses compliance.

Staff use it as their own

Reply as "me" not as the team. Delegation and Send As not configured.

Tidy Up Your Mailboxes

We audit Microsoft 365 tenants, rationalise shared mailboxes, fix delegation, and document the setup for ongoing management.

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