If it's 150,000 recipients, with SMTP_MAX_RCPTS = 900, then that's really just 167 messages, each addressed to multiple recipients (up to 900).
For the record, I must admit it was a huge mistake, because the SMTP connexion is reset after 15 minutes. So Mailman backs off, and waits, and almost nothing comes out. After 5 hours, I had to stop the whole thing, and kill the mail. Now I'm cleaning :(