Hello winkcchiefs,
We understand you're wanting to hide email address using Bcc on your Mac with Mail, right?
If we have that correct, let's first confirm that you have the latest supported macOS version installed. If you need assistance with updating your Mac, check out the following resource: Update macOS on Mac - Apple Support
If you have the latest supported macOS version installed, let's see if the following steps help with what you may be experiencing. You can also locate additional information that may help, here: Address emails in Mail on Mac - Apple Support
Hide email addresses using Bcc
You can help protect the privacy of your recipients by sending your message so that “Undisclosed-recipients” is shown in the To field, instead of everyone’s email addresses.
1. In the Mail app
on your Mac, make sure the Bcc (Blind carbon copy) field is shown in the message window.
If the field isn’t listed, click the Header Fields button
in the toolbar of the message window, then choose Bcc Address Field.
2. Type your recipients’ addresses in the Bcc field.
You can leave the To field blank.
We hope that's able to help. If not, let us know which steps you've taken so far, along with your macOS version. We'll use these details, and any additional helpful information, to continue checking into this with you.
Take care!