cannot send email from our email server to outlook.com

The error message you received indicates that your email server's IP address (148.66.59.235) is on a block list used by Outlook.com. This typically happens when the IP address has been associated with sending spam or other malicious content. To resolve this issue, you should:

1. Contact your Internet Service Provider (ISP): Inform them that your IP address is blocked and ask for assistance in resolving the issue.
2. Check your IP reputation: Use tools to check if your IP address is listed on any block lists and take necessary actions to improve its reputation.
3. Follow the instructions provided in the error message: The message suggests referring to the troubleshooting link, which may provide additional steps for your ISP to follow.

Additionally, ensure that your email practices comply with best practices for email deliverability to prevent future issues.

References:

• Email non-delivery reports and SMTP errors in Exchange Online
• External senders - Use the delist portal to remove yourself from the blocked senders list and address 5.7.511 Access denied errors

Hello

This means the IP for your domain is on the block list due to reasons such as active spam activities on that range, bad reputation, etc. You can contact your host/ISP to see if they can move it to a different IP range, or see if Microsoft would consider delisting (no guarantee): https://sender.office.com/