Microsoft 365 / User mailboxes, shared mailboxes, and aliases: which should you use?

Use a user mailbox for one employee’s email and calendar. If multiple people need to read and respond to mail at the same address, use a shared mailbox. If one person needs to receive mail at another address in addition to their existing mailbox, use an alias. The three methods differ in inbox count, login method, and sending permissions.

Differences between mailbox and alias

Looking at Microsoft’s Exchange Online recipient types and guidance on adding email addresses, messages received via an alias arrive in the existing mailbox. An alias is not a solution for situations that require an independent recipient list or a separate mailbox.

What should be chosen depending on the situation?

• An employee needs a personal email: Use a user account and a user mailbox. Plan offboarding and handover so that personal data is handled separately.
• Addresses like support@ are handled by multiple people: Consider a shared mailbox. Reading and sending permissions need to be set individually.
• It is sufficient if sales@ arrives at an existing address of one person: You can add an alias to an existing user mailbox. If the responsible person changes frequently or joint handling is required, a shared mailbox may be more suitable.
• Distribute announcement emails to multiple recipients: Consider whether a mail group is more appropriate than a separate mailbox.

Can the main address be managed by one person?

For example, if "event inquiries" are handled by only one employee and there is no need to transfer past emails separately, you can start with an alias of that employee's mailbox. However, if the person in charge changes shifts or multiple people need to reply, having mails from the alias mixed in with the personal mailbox becomes a problem. In this case, managing work history in an independent shared mailbox is more suitable for the purpose.

Even if you move the alias to another employee, the past messages from the original mailbox do not move along with it. Distinguish between the fact that the address is the same and the fact that the stored data is the same. Whether frequent staff changes occur and whether customer response records need to be continuously kept are criteria for making choices as important as the cost.

If you only need to distribute messages to several people, also compare distribution groups and Microsoft 365 groups. Delivering a copy to each person differs from working together in a shared inbox. A shared mailbox is not a full help-desk system with task assignment and duplicate-reply prevention, so the people handling it need agreed processing rules.

Is the license for a shared mailbox always free?

No. Microsoft’s shared mailbox guidance explains that a shared mailbox can be used without a separate mailbox license under the basic conditions, but additional capacity and features such as archiving and litigation hold can require licensing. Members also need their own user accounts and the required service entitlements. Check actual usage and current licensing terms before making a decision.

Specifically, the default storage limit for a shared mailbox without a separate license is 50 GB, and each user accessing it must use their own licensed Exchange Online mailbox. To exceed 50 GB and use up to the 100 GB limit, assign an Exchange Online Plan 2 license to the shared mailbox. Archive mailboxes, litigation hold, and advanced compliance features each have separate conditions to check. Do not simply calculate a budget thinking "it's free because it's a shared mailbox."

Keep direct sign-in blocked for the user account associated with a shared mailbox. Instead of sharing the password of the shared account with others, grant delegate permissions to each person's own account. Do not grant shared-mailbox access to external personal email accounts as if they were internal delegates.

Check the current address type

1. Search for the address in Exchange Admin Center's [Recipients] → [Mailboxes]. Check whether it is a user mailbox or a shared mailbox.

2. Check the primary and additional addresses in the target mailbox's email address list. Having additional addresses does not mean a new mailbox has been created.
3. If it is a shared mailbox, also check delegate permissions. Having only read or management permissions does not mean you can send from that address.

If the address is not in the list, you also need to check whether it is a mail alias or a group address. User mailboxes and shared mailboxes can appear together in the mailbox list, but aliases are found in the address list of the corresponding mailbox. If the desired address is already registered to a different recipient object, a new mailbox with the same address cannot be created immediately.

Add an alias and verify the result

This is the procedure when only an additional email address is needed for an existing user mailbox. You need permission to manage the recipient’s addresses and a verified domain, and record the list of primary and additional addresses before making changes.

1. In Exchange Admin Center → Recipients → Mailboxes, select the target user. Confirm it is the same person by checking the full email addresses.
2. Open General → Email addresses → Manage email address types and check the existing primary address and alias. The menu name may vary slightly depending on translation or screen.
3. Select Add email address type, choose SMTP (Simple Mail Transfer Protocol), and enter the alias and domain. SMTP is the email-address type used for ordinary internet email. Distinguish whether you are changing the default reply address or just adding an alias, then save.
4. Open it again to check if the address has been saved, and send mail from an external test address to the new alias. The expected outcome is that it arrives in the same inbox of the existing mailbox.
5. After replying, also check the sender address displayed to the recipient. Being able to receive mail with an alias does not automatically guarantee sending with that alias. Review organizational settings and support in the Outlook client you are using separately.

If you cannot modify the address in a synchronized account, follow the management procedure of the original directory. If the alias is incorrect, remove only the address added this time and keep the existing primary address. Be aware that you may not receive mail sent to the removed address, so if it is a public address, also consider user/customer guidance and alternative delivery paths.

Things to check before moving the address to another mailbox

In the plan to change the address, include the currently connected entities, the destination to move to, the handover of past emails, sending permissions, and test times together. If you want to convert a user mailbox to a shared mailbox, check the capacity and retention conditions, then handle it by linking with employee email handover procedure (Korean). Do not proceed by deleting the license or user account first.

If you have selected a shared mailbox, set read and send permissions separately in creating a shared mailbox and granting permissions. After the conversion, access to past emails and reception/reply of new emails must be checked separately to determine whether the handover is complete.