This covers a wide area of problems
If you have sent mail to the recipient address in the past, my first guess would be spam filtering at either the ISP or receiver mail client. If it's at the ISP level, messages will never get delivered even to a spam folder. Everyone you contact will need to contact their own provider to painstakingly troubleshoot the problem. You providing them with exact dates and times cab be helpful to all involved. Like a poster before said m make sure you have not been blacklisted, a terrible process to fix if you have been. If you are sending to a company that is using Exchange and an email cannot be delivered, you will receive a bounceback email with av 3 digit code 5xx. You can then search the internet for "exchange error codes" to identify the issue.
These types of email issues are extremely frustrating to deal with and troubleshoot. Best of luck.