Google (gmail.com) mail getting blocked due to bad reputation

Dear @Noyon Chandra Das

Thank you so much for contacting Microsoft Q&A Support. 

You're experiencing a mail sending issue from your Microsoft 365 mailbox to Gmail users, and your domain reputation has been marked as "bad" in Google Postmaster Tools. Based on the image you provided, your domain XXXXX.com dropped from Medium to Bad reputation between May 23 and June 1, and only recently recovered to Medium by June 13. This reputation drop is likely the reason Gmail is rejecting or marking your emails as spam (maybe related to send bulk emails). 

Please understand that our forum is a public platform, and we will modify your question to hide the image with your domain name in the description. Please notice to hide these personal or organization information next time you post error or some information to protect personal data. 

As a result, I research and find several suggestions that you can try: 

1. Verify and Configure Authentication Records  Ensure your domain has properly configured: 

• SPF (Sender Policy Framework) 
• DKIM (DomainKeys Identified Mail) 
• DMARC (Domain-based Message Authentication, Reporting & Conformance) 

References:  

• Set up SPF identify valid email sources for your Microsoft 365 domain - Microsoft Defender for Office 365 | Microsoft Learn 
• How to use DKIM for email in your custom domain - Microsoft Defender for Office 365 | Microsoft Learn 
• Use DMARC to validate email, setup steps - Microsoft Defender for Office 365 | Microsoft Learn 

1. Clean Your Email Lists: you should avoid sending emails to invalid or inactive addresses. High bounce rates and low engagement hurt your reputation. 
2. If you want to send bulk emails, you should practice: 

• Warm-up your IP address: To establish a good sender reputation, start by sending a low volume of emails and gradually increase the count as you establish a pattern of safe and wanted emails. 
• Segment Your Email List: Group your recipients based on common characteristics to send more targeted emails. 

See this article to consult more details: 

• Issues with Sending Bulk Emails to Gmail Users via Outlook - Microsoft Q&A 
• Sending limits in Outlook.com - Microsoft Support 

If the reputation of your domain currently has this bad status, I advise you should improve it by trying my suggestions. Also, to avoid it in the future, you can take a look these tips: 

1. Avoid Sending Bulk Emails in high volume: you should gradually increase your sending volume if you’ve recently started sending bulk emails because sudden spikes can trigger Gmail’s spam filters. 
2. Maintain a Consistent Sending Pattern to avoid sudden spikes in volume or frequency. 
3. Practicing for deliverability, see this article: Authenticate Outbound Email to Improve Deliverability | Microsoft Community Hub 

I hope this information can help you to solve it. 

Wish you a pleasant day! 

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