User having Storage Space Warnings but still has space

Hi @Jeremy A. Snearl,

Based on your information, the issue is likely due to that the user's mailbox reaches the ProhibitSendReceiveQuota. The 85% in mailbox storage space is only the visible mailbox usage, not the actual size which includes the hidden Recoverable Items folder.

If you are an administrator, you can follow these steps to resolve this issue. I will summarize the steps shortly, so if you want to read full detailed guidance, please refer to these articles:

• Demystifying Exchange Online Mailbox Quotas | Microsoft Community Hub
• (Mailbox is full) warning when it hasn't reached quota - Microsoft 365 | Microsoft Learn

1/ Check the actual quota values on the mailbox

Open Exchange Online PowerShell and run:

Get-Mailbox <user> |fl Alias, *Quota

In the listed quota, mainly check these three:

• ProhibitSendReceiveQuotas: this is the primary mailbox quota. When the primary mailbox of the user reaches this value, the user won’t be able to send and receive emails.
• ArchiveQuota: the archive mailbox quota. When the archive mailbox (or mailbox shard) of the user reaches this value, we cannot add more content to that specific archive mailbox (or shard).
• RecoverableItemsQuota: the recoverable items quota. When recoverable items in the primary or archive mailbox shard reaches this value, the user won’t be able to delete from Deleted Items or move content to Recoverable Items folders.

 2/ Fix the issue based on the quota status

• If primary mailbox is near or at the limitation: empty Deleted items, remove large message/attachments or archive older mails
• If Recoverable items folder is the problem: verify if an archive mailbox is enabled, then run the Managed Folder Assistance to process retention and clear/move eligible items

Hope my sharing provides clear details for helping with your concern. If you have any questions or need further assistance, please feel free to share them in the comments on this post so I can continue to support you.    

Thank you for your understanding and cooperation. I'm looking forward to your reply.

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Conference invitations can be rejected even when the mailbox is below 100% if either:

1. The mailbox is close enough to its quota that warning or send/receive limits have kicked in, or
2. The problem is not the mailbox quota but the user’s Microsoft cloud storage (OneDrive + attachments) being over limit.

For work/school accounts in Outlook on the web / new Outlook:

• Mailboxes can be configured by the admin with lower limits than the maximum 100 GB. Once the configured quota is exceeded, the user cannot send or receive any messages, including meeting invitations, and senders get a “mailbox full”/quota error.
• Admins can reduce maximum mailbox sizes per user or globally, so the effective limit for this user might be lower than expected.

For Microsoft storage (if this is a consumer-type mailbox or unified storage applies):

• If Microsoft cloud storage is exceeded, Outlook.com cannot send or receive email even if the email storage itself is not full. This also blocks messages with calendar invites until storage is back under quota.

Actionable checks and fixes:

1. Verify the actual mailbox quota and thresholds
   • In Outlook on the web, go to Settings > Account > Storage (or search for Storage in Settings) to see:
     • Total mailbox quota
     • Exact usage percentage
   • If the mailbox is near the configured limit (not just 85% of 100 GB), clean up or archive mail:
     • Use Settings > Storage to see folder sizes and bulk-delete items older than 3/6/12 months.
     • Empty Deleted Items, as it still counts toward the quota until emptied.
2. Reduce mailbox size
   • Delete large messages and attachments, especially in Inbox, Sent Items, and Deleted Items.
   • Use Sweep in Outlook on the web to remove bulk mail from specific senders, then empty Deleted Items.
   • Move items to an archive mailbox if available.
3. Check Microsoft/OneDrive storage (if applicable)
   • If this is a Microsoft account using unified storage, being over the Microsoft storage quota blocks sending/receiving email even when email storage is under its limit.
   • Free up OneDrive/cloud storage (delete or move files, empty recycle bin) until total Microsoft storage is under quota.
4. If limits are too tight
   • As admin, review and, if appropriate, increase the user’s mailbox quota in Exchange Online so normal mail and calendar traffic (including invites) is not blocked prematurely.

Once both mailbox storage and Microsoft/unified storage are under their respective quotas, new conference invitations should be delivered normally.

References:

• Mailbox storage limits in Outlook
• Storage limits in Outlook.com