Cannot access Exchange Online mailbox – Microsoft 365 tenant and account issue

Adriána Mojžišová 0 Reputation points
2026-11-04T11:05:07.2066667+00:00

Hello Microsoft Support Community,

I need assistance with my Microsoft 365 and Exchange Online configuration.

My situation:

  • My email address is [Moderator note: Personally Identifiable Information removed]@nikel.sk.
  • My domain nikel.sk is already registered in the Microsoft 365 tenant nikelsk.onmicrosoft.com.

I have access to the Microsoft 365 Admin Center for the existing organization.

An Exchange Online Plan 1 license is assigned to my user account.

However, I cannot access my mailbox through Outlook on the web.

Microsoft displays an access or authentication error when I try to sign in.

I have also created another Microsoft 365 tenant and purchased an additional Exchange Online Plan 1 license.

Important information:

My existing mailbox is currently hosted by an external provider (mail.unio.sk) using IMAP. It is still working correctly, and I have not changed the MX records.

I need help with the following:

Verify whether an Exchange Online mailbox has been provisioned for [[Moderator note: Personally Identifiable Information removed]@nikel.sk] in the existing tenant.

Identify why Outlook on the web denies access despite the assigned license.

Verify the account authentication and permissions.

Determine the best way to use my existing tenant and avoid duplicate Exchange Online subscriptions.

Prepare a safe IMAP-to-Exchange migration without losing existing emails.

I am the owner of the domain and can verify its ownership.

Thank you for your assistance.

Exchange Online
Exchange Online

A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.

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  1. BabybooHN 3,810 Reputation points Independent Advisor
    2026-11-04T14:57:51.9866667+00:00

    Dear @Adriána Mojžišová

    I recommend troubleshooting the mailbox in onmicrosoft.com first, since your domain and licensed user already exist there. Keep your current MX records unchanged while resolving access. MX records control incoming mail delivery; they do not prevent a provisioned Exchange Online mailbox from opening in Outlook on the web.

    Please check the following:

    1. Confirm the account and license. In Microsoft 365 Admin Center⁠, verify that you are viewing the original organization. Go to Users > Active users > your user > Licenses and apps, and confirm that Exchange Online Plan 1 is assigned and its Exchange service is enabled. Note the user’s actual sign-in username, which may differ from the email address.
    2. Check whether the mailbox exists. Open Exchange Admin Center⁠ > Recipients > Mailboxes and search for the user. If it is missing despite valid licensing, follow Microsoft’s mailbox provisioning guidance⁠. If the issue persists after 24 hours, open a support request through Admin Center > Help & support.
    3. Test the correct sign-in. Open an InPrivate/Incognito window, visit https://outlook.office.com/mail/, and sign in with the original tenant’s user credentials. If prompted, select Work or school account. The password at your external IMAP provider may be different.

    For migration, first back up the existing mail and confirm the destination mailbox works. Then configure an IMAP migration from your provider, verify the migrated messages and synchronization results, and change MX records only when ready for cutover. Keep the source mailbox available until delivery and final synchronization are verified. IMAP migration excludes contacts, calendars, and tasks, which need separate handling.

    For the additional subscription, review Billing > Your products in the second tenant. Once you confirm it is unnecessary and contains no required data, cancel it according to its subscription terms.

    I cannot inspect your tenant directly. Please share the exact Outlook error text/code, whether the mailbox appears under Recipients > Mailboxes, and when the license was assigned. Hide personal information in any screenshots.

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