someone is using my icloud email address that I've had for years what can I do?

When you tried to send the mails from your IMAP or web iCloud account the mails were bounced ? why it means that is a spoof mail address .

They can't exactly copy and paste your mail address and use it as their own , create same prefix with iCloud domain mail address , as for an every individual iCloud account the credentials are confined , even they can't create an alias for that mail .

The reason when you created an Apple ID account it could be with secondary authentication account the security answers and questions and answers , date of birth , rescue mail address and its password was created by you , these are secrets and never shared with others , so it's impossible that second person can use your credentials to create same Apple ID address ( it also applies for gmail accounts ) .

However , in any case you were sending mails to that person and he knows your mail address they can send you blank mails in which they don't add any subject and simply use mail address , when mail arrives it will be empty in your icloud.com account or IMAP account if configured https://support.apple.com/en-in/HT211161

https://support.apple.com/lt-lt/guide/mail/cpmlprefcomp/mac

It's not necessary they are using Mac , it could be windows computer , for you the question is from iCloud.com https://support.apple.com/en-in/guide/icloud/mm6b1a5a47/icloud

It’s possible that that individual could’ve simply mixed up the letters in your email as theirs may be close to yours, and they didn’t realize it as the emails intended for them aren’t coming to them.  Are there too many emails that you could possibly reply to the sender that it’s not gong to the intended receiver?

How do you know someone is using your iCloud email address?