Exchange Server admin: GMail spam filter rejects bulk messages

It seems that your bulk emails to Gmail users are being rejected due to rate limiting, which is a common issue when sending a large volume of emails from a single domain. The error messages you are seeing, particularly the 550 5.4.300, indicate that Gmail has detected an unusual rate of mail originating from your domain, leading to temporary restrictions on your sending ability.

Here are some steps you can take to address this issue:

1. Review Sending Practices: Ensure that your email sending practices comply with best practices. Avoid sending too many emails in a short period, as this can trigger spam filters.
2. Authenticate Your Emails: Make sure that you have proper DKIM, SPF, and DMARC records set up for your domain. This helps establish your domain's legitimacy and can improve deliverability.
3. Monitor Bounce Rates: Keep an eye on your bounce rates and remove any invalid email addresses from your list. High bounce rates can negatively impact your sending reputation.
4. Segment Your List: If possible, consider segmenting your email list and sending emails in smaller batches over a longer period. This can help prevent Gmail from flagging your emails as spam.
5. Contact Google Support: If the issue persists, consider reaching out to Google support for further assistance. They may provide insights specific to your situation.
6. Check NDR Details: Review the Non-Delivery Report (NDR) for any additional information that might help you understand the issue better. It may contain specific guidance for email admins.

By following these steps, you should be able to improve your email deliverability to Gmail users and reduce the likelihood of encountering similar issues in the future.

References:

• Fix NDR error "550 5.1.10" in Exchange Online
• Email non-delivery reports and SMTP errors in Exchange Online

Hello Boris Andreev
Thank you for posting question to Microsoft Q&A Forum.
Please understand that our forum is a public platform, and we will modify your question to cover your organization's domain name and your in the description. Please note that you should hide this personal or organizational information the next time you post a bug or some information to protect personal data.
According to this issue, please check if you've set up your DKIM based on this article (DKIM should be set up in Microsoft 365 Security Center - https://security.microsoft.com/ https://learn.microsoft.com/en-us/defender-office-365/email-authentication-dkim-configure?view=o365-worldwide#steps-to-manually-set-up-dkim
You shouldn't be using Office 365 to send bulk emails. Please consider using a proper service that will handle bounces and unsubscribe requests for you automatically, which a manual list on Office355 cannot do. Someone probably wants off the list, and as there is no one click unsubscribe, they are marking the messages as spam.
You could consider using this Postmaster Tool (from Google) and monitor if this could improve the situation: https://support.google.com/mail/answer/6227174?sjid=7925206490225307693-NC
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
I would suggest that you could also raise a ticket to Microsoft 365 Admin Center (Microsoft Support Team), they could look into your email patterns and check on them. As I am forum moderator, I do not have access to Microsoft internal system to check this for you.
Please consider checking both suggestions above and monitor to see if the situation is getting better. I am pretty sure that this issue could not be immediately resolved.
I hope this information is helpful. Please let me know if you have any questions.

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