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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.

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Yes it is, ISP's measure your performance on multiple levels. One is the amount of bad mailboxes you are trying to deliver to. Different ISP's of course measure different things but people like AOL and yahoo do use this as a metric.

Other metrics include whether you are using domain key or SPF authenticated mail. How many of your users report your mail as "spam". Some of the more technologically advanced ISP's are even looking at engagement metrics now (whether people open/click your emails) to decide whether they will deliver your email at all, to bulk or to the inbox.

2% bounce rate however is an acceptable level of bounces provided your other factors are good, make sure you keep it below 5% or you will have inbox delivery issues.

N.B If other people are using the same IP address to deliver their email they could also be impacting your delivery.

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    Hotmail also use old, abandoned email addresses as spam traps and share their results with SenderScore who handle reputation scoring (and blacklisting) for many other domains. AOL seem to use these to throttle your sending rate rather than blocking your emails entirely.
    – Ladadadada
    Feb 24, 2012 at 11:30

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