Hello @Syed Afsar Afsar,
Welcome to the Microsoft forum for technical support.
Based on your description, I have a few questions to confirm with you:
- Are you using Exchange Server?
- Because many email systems prevent you from sending emails, are your SPF, DMARC, and DKIM records set correctly?
If your settings are correct, it may be that your IP address has been blocked. You can choose the following methods to remove your IP address from block list:
1.For Outlook.com or Hotmail.com accounts, you can fill in the relevant information through this link: New support request (office.com) to seek to remove your IP address from the block list.
2.If you are also having issues sending to recipients with email addresses in Microsoft 365, you can use the removal portal (Delist IP - Delist IP (office.com)) to remove yourself from the blocked senders list delete.
Please feel free to contact me if you have any queries.
Best,
Jake Zhang