First check both Outlook.com mailbox storage and Microsoft cloud storage, because exceeding either quota prevents incoming mail and causes messages to be returned to senders; however, the complete bounce message is needed to determine whether storage, sender blocking, or another policy produced this generic 550 5.7.1 rejection.^1^
1. Check storage after the renewal
For a personal Outlook.com/Microsoft 365 account:
- Open Outlook.com.
- Go to Settings → Account → Storage.
- Check whether the mailbox is at its limit.
- Also check the Microsoft cloud-storage usage shown for the account, because exceeding that quota can stop Outlook.com mail even when mailbox storage itself is available.^1^
- If storage is full, review or export any important email before deleting it. Then remove large or unnecessary messages and empty Deleted Items as required. Emptying Deleted Items can permanently delete those messages.
- Once usage is below the quota, ask each affected sender to resend their message. Previously bounced messages are not delivered automatically.
An expired consumer Microsoft 365 subscription can reduce the account to its free storage allowance. If usage then exceeds the cloud or mailbox quota, Outlook.com cannot receive mail; renewing restores the subscription benefits, but the account must also show that it is back within its active quota.[^2^](https://support.microsoft.com/accounts-billing/subscriptions/[Subscription ID] "What happens when your Microsoft 365 subscription expires | Microsoft Support")
2. Obtain the complete bounce message
Ask one affected sender to forward the entire non-delivery report (NDR), including:
- All text following
550 5.7.1
- Any code such as
S3150, AS(...), or TRANSPORT.RULES.RejectMessage
- The reporting server and diagnostic section
- The date, time, sender, and recipient domain
The sender should preserve the original NDR privately for their email provider or Microsoft Support. Redact email addresses, IP addresses, message IDs, and other sensitive details before posting it publicly.
The exact wording determines the next action:
- If it says
S3150 or that the sending network/IP is on Microsoft’s block list: the sender or their mail provider must open https://sender.office.com, enter the blocked IP shown in the NDR and the address that received the bounce, and submit a delisting review. If the sender does not control that outbound IP, their ISP or email provider must submit the request.^3^
- If it says the message was identified as spam: this is normally a sender-side reputation, authentication, or content issue. The sender should use Outlook.com Sender Support and have their mail administrator review the sending IP, SPF, and mail configuration.^4^
- If it explicitly says mailbox/storage/quota exceeded: reduce storage as described above, then have the sender resend.
- If none of those phrases appears: do not submit an IP-delisting request yet; the generic
550 5.7.1 code alone does not identify an IP block.
If this is a work or school mailbox
The consumer Outlook.com storage path may not apply. Ask the Microsoft 365 administrator to open Exchange admin center → Mail flow → Message trace, search for an affected sender and delivery time, and inspect whether the status is Failed, Quarantined, or Delivered.^5^
If this is a personal Outlook.com account and storage is below quota, use Help in Outlook.com, enter the error, and select Still need help? → Yes to contact support.^6^
References
- Storage limits in Outlook.com | Microsoft Support
- [What happens when your Microsoft 365 subscription expires | Microsoft Support](https://support.microsoft.com/accounts-billing/subscriptions/[Subscription ID])
- How to I contact Microsoft about Block List S3150 - Microsoft Q&A
- Sender Support in Outlook.com
- Error When Receiving Emails - Microsoft Q&A
- Can't send or receive email in Outlook.com | Microsoft Support