Microsoft brings strict rules for bulk sending of Outlook emails

Microsoft has officially rolled out a major update that could cause email administrators headaches. As of today, Microsoft has begun rejecting bulk Outlook emails that don’t comply with its strict authentication standards.

Microsoft cracks down on sending bulk Outlook emails with strict rules

If you send over 5,000 messages daily to Outlook.com, Hotmail.com, or Live.com addresses and your domain isn’t correctly configured with SPF, DKIM, and DMARC, your emails won’t just land in the spam folder — they’ll be outright blocked. Microsoft confirmed this change will reduce spam, phishing, and spoofing attacks.

Also read: Microsoft to fix CPU spike bug plaguing Classic Outlook in May

Microsoft’s strict policy for sending bulk Outlook emails doesn’t just affect spammers — it could impact legitimate senders like marketers, businesses, and newsletter publishers. According to Microsoft, all three protections must be in place:

• SPF must specify authorized sending IPs
• DKIM must cryptographically sign messages
• Published DMARC must align with SPF or DKIM

In the announcement post, Microsoft stated:

After careful consideration and to ensure the protection of users and remove any confusion on why a message was in the junk folder for both the recipient and sender, we have made a decision to reject messages that don’t pass the required authentication requirements detailed above. The rejected messages will be designated as “550; 5.7.515 Access denied, sending domain [SendingDomain] does not meet the required authentication level.”

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Third-party email platforms don’t guarantee protection. Misconfigured DNS records, outdated contact lists, or too many SPF DNS lookups can all trigger blocks. That’s not all; being on a user’s Safe Senders list offers no immunity.