As mentioned in a previous question of mine, I send a lot of emails.
I have calculated that approximately 2% of them are coming back to me immediately with the general text reading:
550-5.1.1 The email account that you tried to reach does not exist. Please try 550-5.1.1 double-checking the recipient's email address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1 http://support.google.com/mail/bin/answer.py?answer=6596
That above text is from Gmail, but many other email providers respond with similar text.
My question is: Is it dangerous (in terms of being blacklisted) to send email to addresses that do not exist?
I plan on creating a cron job shortly that removes users who have not logged into the site in the past month from the newsletter, but perhaps I won't bother if there are no negative effects.