Microsoft has tightened enforcement against bulk or marketing-style email sent directly from individual Microsoft 365 or Exchange Online mailboxes. Identical or near-identical messages, especially those with multiple links or tracking, are now far more likely to be throttled, delayed, or blocked. When this happens, senders may be temporarily restricted and administrators receive alerts. This article explains why these restrictions occur and how to send messages successfully.
Applicable to: Fort Collins, Pueblo, Spur, System
Affiliation: Faculty, Staff, Student employees
Prerequisites
Before sending a large or repeated email to many recipients, coordinate with your local IT staff to determine the best method for your audience and message type.
Features
What changed
- Microsoft now more aggressively detects and limits mass mailing patterns from user mailboxes, even if they aren’t compromised.
- Suspicious outbound traffic can be rerouted through the high-risk delivery pool or the sender can be restricted.
- CSU’s Division of IT (DoIT) is aligning with these protections, exploring lower daily/hourly caps to limit potential security and compliance risks.
Typical symptoms
- Bounce errors such as “550 5.1.8 Access denied, bad outbound sender AS(45007)” or “restricted user” alerts.
- Mail sends but is routed through the high-risk delivery pool, landing in spam or being rejected by external providers.
Common triggers
- Large numbers of recipients in a short period.
- Identical or near-identical message content to many people.
- Link-heavy templates, URL shorteners, or trackers.
- Forwarding or relay issues and misaligned DNS (SPF, DKIM, DMARC).
Recommended ways to ensure successful delivery
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Batch small sends (short-term workaround)
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Send smaller batches (around 200 recipients per day for a week) to avoid hourly/daily caps.
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Coordinate with local IT if your account was previously restricted.
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Why this works: It avoids “spiky” behavior flagged by Microsoft and stays within tenant thresholds.
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Ask local IT to use smtp.colostate.edu for programmatic mail (IT-assisted)
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Request assistance from your local IT team for departmental announcements or operational notices.
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Use scripting options such as PowerShell or Python for controlled, logged, and paced mail sends.
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Use a proper marketing platform (best for large or recurring sends)
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Choose platforms such as Mailchimp, Constant Contact, Slate, Salesforce Marketing Cloud, or ThankView.
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Follow IT guidance for DNS and DMARC setup at Third-party mailers guidance .
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Why this works: These tools manage consent, unsubscribe options, and ensure higher deliverability.
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Send to a group instead of many individuals
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Request that local IT set up a Listserv, Distribution List, Microsoft 365 Group/Team, or mail-enabled security group.
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Note: Large groups still count per recipient after expansion, but group sends reduce per-message volume and are easier to manage.
Which option should I choose?
- I need to reach 150–500 people once: Use a group or batch your sends.
- I need to reach 1,000+ people or send regularly: Use a marketing platform or groups.
- It's departmental or IT-assisted: Use smtp.colostate.edu.
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Not sure? Open a ticket with local IT and share your audience size, frequency, and message details.
Best practices for deliverability
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Avoid URL shorteners and limit the number of distinct links.
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Personalize the subject and body slightly to reduce identical message patterns.
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Use approved sender addresses and aligned subdomains (SPF/DKIM/DMARC).
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Test with a small pilot list before sending widely.
What to do if you’re already blocked
If you receive a bounce like AS(45007) or can’t send mail, contact your IT support lead. They can review message traces, alerts, and coordinate mitigation with second-level support.
Why we’re doing this
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Security and reputation: One compromised account can affect the entire university’s deliverability and reputation.
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Right tool for the job: Microsoft 365 mailboxes aren’t designed for bulk mail. Dedicated tools ensure compliance, analytics, and domain health.
Helpful links
Outcome
Faculty and staff will understand why bulk messages from Microsoft 365 are restricted and how to choose the correct method for sending large or recurring email communications successfully.
Suggested Tags: mass-email,Microsoft-365,email-blocking