SMTP errors · SimpleVerifier Team · checked 2026-10-30
Outlook and Microsoft 550 5.7.515 and the bulk sender rules
550 5.7.515 is Outlook.com refusing mail from a domain that sends more than 5,000 emails a day to Outlook.com, Hotmail and Live addresses without passing SPF, DKIM and DMARC. Microsoft has enforced the rule since 5 May 2025. The fix is your domain’s DNS, not your list. Every other Microsoft rejection, from banned IPs to recipient-not-found, is grouped below with its own page.
Source: Microsoft: Outlook's new requirements for high-volume senders (2 Apr 2025, updated 30 Apr 2025), read 2026-10-30. Check your own domain with the Microsoft sender requirements checker.
Microsoft’s bulk sender rules
They apply to domains sending more than 5,000 emails a day to Outlook.com consumer addresses (outlook.com, hotmail.com, live.com). Microsoft’s own wording:
| Check | Requirement | Tool |
|---|---|---|
| SPF | "Must Pass for the sending domain." | SPF lookup counter |
| DKIM | "Must Pass to validate email integrity and authenticity." | DKIM signature validator |
| DMARC | "At least p=none and align with either SPF or DKIM (preferably both)." | DMARC alignment checker |
Microsoft also recommends, without making them requirements, a valid From or Reply-To address that can receive replies, a working unsubscribe link, removing invalid addresses regularly, and recipients who consented. It says it may filter or block senders who ignore them. The Safe Senders list is not honoured for this enforcement.
The rule covers Outlook.com consumer mailboxes. Mail to Microsoft 365 business tenants is filtered by each tenant, and its rejections carry different codes, listed below.
Outlook.com, Hotmail and Live rejections
Microsoft's consumer service. 5.7.515 is the bulk sender rule; S3140 and 5.5.0 are its block-list and mailbox-unavailable replies.
- 550 5.7.515
authentication required for high-volume senders (Outlook.com)
SMTP 550 5.7.515 is Outlook.com rejecting mail from a high-volume sender that does not meet its authentication requirements.
- Outlook.com S3140 (550 5.7.1)
IP range on Outlook.com block list
Outlook.com error S3140 is a 550 5.7.1 rejection meaning part of your sending network is on Microsoft's consumer block list.
- 550 5.5.0
Mailbox unavailable (Outlook.com, Hotmail)
550 5.5.0 "Requested action not taken: mailbox unavailable" is what Outlook.com returns when a Hotmail, Outlook or Live address does not exist.
Authentication and policy (Microsoft 365)
The recipient's tenant refused the message on SPF, DMARC or its own sender rules. Fixed in your DNS or by the recipient's admin.
- 550 5.7.23
SPF violation (Microsoft 365)
SMTP 550 5.7.23 is Microsoft 365 rejecting a message because of an SPF failure.
- 550 5.7.509
DMARC failed, policy reject (Microsoft 365)
SMTP 550 5.7.509 means Microsoft 365 rejected the message because the domain in its From header failed DMARC, and that domain publishes p=reject.
- 550 5.7.12
Sender not authenticated by organization (Microsoft 365)
550 5.7.12 means the recipient's Microsoft 365 address only accepts mail from inside its own organisation, so mail from outside is rejected.
- 550 5.7.124
sender not in allowed senders list (Microsoft 365)
SMTP 550 5.7.124 means you sent to a Microsoft 365 distribution group that only accepts mail from listed senders, and you are not on that list.
- 550 5.7.513
Blocked by the recipient's own block list (Microsoft 365)
550 5.7.513 means the recipient's organisation added your sending IP to its own block list in Microsoft 365.
Your IP, account or tenant is blocked
Microsoft blocked the sender, not the recipient. Delisting or an admin release is the fix; every message fails until then.
- 550 5.7.606
banned sending IP (Microsoft)
SMTP 550 5.7.606 to 5.7.649 means Microsoft has put your sending IP on its block list.
- 550 5.7.705
tenant has exceeded threshold (Microsoft 365)
SMTP 550 5.7.705 means Microsoft has blocked outbound mail from your whole Microsoft 365 tenant.
- 550 5.7.708
traffic not accepted from this IP (Microsoft 365)
SMTP 550 5.7.708 means Microsoft 365 will not relay mail from your tenant's sending IP because that IP has a low reputation.
- 550 5.7.750
blocked from sending from unregistered domains (Microsoft 365)
SMTP 550 5.7.750 means Microsoft 365 has blocked your tenant because it sent a large amount of mail from domains that are not registered as accepted domains in the tenant.
- 550 5.1.8
bad outbound sender (Microsoft 365)
SMTP 550 5.1.8 means Microsoft 365 has blocked your own account from sending.
- 451 4.7.500
access denied, try again later (Microsoft 365)
SMTP 451 4.7.500 to 4.7.699 (shown with an ASxxx tag) is Microsoft 365 temporarily throttling mail from your IP.
- 530 5.7.57
client not authenticated (Microsoft 365)
SMTP 530 5.7.57 is Microsoft 365 refusing mail from a device or app that connected to smtp.office365.com without signing in.
The address does not exist
Hard bounces from Microsoft-hosted domains. These are the ones verifying the list before you send prevents.
- 550 5.4.1
recipient address rejected, access denied
SMTP 550 5.4.1 is Microsoft 365's rejection for an address it does not recognise or will not accept mail for.
- 550 5.1.10
recipient not found (Microsoft 365)
SMTP 550 5.1.10 is Exchange Online's "recipient not found" error: Microsoft looked up the recipient's address and found no mailbox for it.
- 550 5.1.0
address rejected, other address status
SMTP 550 5.1.0 means the receiving server rejected the address without giving a more specific reason.
Where list verification helps, and where it does not
Authentication codes (5.7.515, 5.7.23, 5.7.509) and IP or tenant blocks are fixed in DNS and with Microsoft, not by cleaning a list. The recipient-not-found codes are the ones verification prevents: Microsoft 365 answers mailbox checks for most tenants, so those addresses come back invalid before the send. Outlook.com, Hotmail and Live mailboxes are a different case: Microsoft is currently refusing mailbox checks from our server’s network, so those rows come back risky (mailbox unconfirmed), never valid.
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Common questions
- What does 550 5.7.515 mean?
- Outlook.com rejected mail from a domain that sends more than 5,000 messages a day to Outlook.com, Hotmail and Live addresses and does not meet Microsoft's authentication requirements. Microsoft's rejection text is "550; 5.7.515 Access denied, sending domain [SendingDomain] does not meet the required authentication level." Fix SPF, DKIM and DMARC for the domain it names.
- What are Microsoft's bulk sender requirements?
- For domains sending more than 5,000 emails a day to Outlook.com consumer addresses: SPF must pass, DKIM must pass, and DMARC must be at least p=none and align with SPF or DKIM. Microsoft announced them on 2 April 2025 with enforcement from 5 May 2025, and also recommends a valid From or Reply-To address, a working unsubscribe link, removing invalid addresses regularly, and consented recipients.
- Do the Outlook bulk sender rules apply to Microsoft 365 business mailboxes?
- Microsoft's announcement says it applies to Outlook.com, its consumer service for hotmail.com, live.com and outlook.com addresses. Microsoft 365 business tenants have their own filtering and their own codes, such as 5.7.23 for SPF failures and 5.7.509 for DMARC rejects.
- Will verifying my list fix 550 5.7.515?
- No. 5.7.515 is an authentication failure on your sending domain, so the fix is DNS. Verification prevents a different set of Microsoft bounces: 5.4.1, 5.1.10 and 5.5.0, where the mailbox does not exist. Microsoft's own hygiene recommendation is to remove invalid addresses regularly.
- Does adding the sender to Safe Senders get around the rule?
- No. Microsoft's FAQ on the announcement says the Safe Sender list won't be honored for this enforcement.