Are there any NDR error messages when email invites blocked?
What is your environment now? Exchange Online or hybrid environment?
If you send normal emails to external organization, can they receive it normally?
It sounds like you have already done some troubleshooting to identify the cause of the issue. One thing you could try is checking if your domain is on any email blacklists. You can use a tool like MX Toolbox to check this.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.