How do I use the mail manager without the $to value? I want to send an email for multiple users without a to address. Setting $to NULL, and empty string 'undisclosed-recipients:;' doesn't work.
How can I send an email with just BCC recipients?
I would suggest that you avoid this approach as messages with lots of recipients in the bcc field are more likely to trigger spam filters on the receiving mail server.
I would loop through each email address and send them a separate email instead, if there are a lot of emails to send then consider moving them into a batch operation.