Why are a few clients emails being blocked

Question

Why are a few clients emails being blocked

Lisa Higgins 0

All of a sudden people that have emailed for years are getting a "Message Blocked" Error when they try to email me. I am still getting other emails but 3 have reached out this week with this error

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2 answers

Your answer

Answer 1

If only a few people are getting Message Blocked while other mail still arrives, the problem is usually on the sender side, not with the mailbox.

Common causes supported here:

The sender’s server or IP is blocked or has a poor reputation
- This can happen if the sender’s mail provider or IP range has recently been associated with spam or abusive activity.
- In that case, some senders can reach the mailbox while others are rejected.
The sender’s domain authentication is failing
- If the sender fails SPF, DKIM, or DMARC checks, their message can be treated as suspicious and blocked.
- This must be fixed by the sender or their email provider.
The sender is blocked by Microsoft 365 protection
- External senders can receive access denied or blocked sender errors when their sending IP is on Microsoft’s blocked senders list.
- Those senders need to follow the delisting process shown in the NDR they received.

What to do:

Ask each affected sender to send the full bounce message/NDR.
Check whether the error mentions:
- blocked sender / banned sending IP / access denied
- SPF fail
- a blocklist or reputation issue
If the NDR shows a blocked IP or sender reputation problem, the sender must contact their own email provider/ISP to resolve it.
If the NDR shows SPF or other authentication failure, the sender must correct their domain mail authentication settings.
If the sender’s NDR specifically says they are blocked by Microsoft 365, they can use the Microsoft delist process described in the NDR.

Because other messages are still being delivered, this does not match a mailbox storage problem or a general Outlook client problem.

Answer 2

Hello

I am seeing a sudden increase in similar reports from other users in this forum as well.

Unfortunately, I cannot find any official information from Microsoft acknowledging this issue. Microsoft's published requirements for SPF, DKIM, and DMARC apply to senders sending more than 5,000 emails per day. However, based on the sudden increase in reports yesterday, I suspect these requirements may now be affecting low-volume senders as well. This is just my observation, not something Microsoft has officially confirmed.

I helped several users with this issue yesterday in this forum. One interesting thing we noticed is that email headers may show DMARC as passing, even when the sending domain does not have a published DMARC record. We were able to identify this by checking the domain using a free tool such as https://mxtoolbox.com/.

In some cases, once the missing DMARC record was properly configured and verified, emails started coming through again.

For this reason, I strongly recommend having your clients check their SPF, DKIM, and DMARC records for the affected sending domains. Another useful tool is https://dmarcian.com/domain-checker/.

If you need help checking these records, I am happy to do so. I would just need to know the sending domain name. You do not need to share the full email address.

For now, verifying and correcting these authentication records is the best troubleshooting step I can recommend until Microsoft provides more official information.

Note: These are non-Microsoft websites. They appear to provide useful information, but be cautious of advertisements or downloads for potentially unwanted software. You do not need to download anything to use the domain-checking tools.