I know this email address is valid, but it hard bounced.

Valid email addresses can hard bounce for many reasons. Learn to read bounce headers (SMTP replies) to get some insight on what caused an address to hard bounce.

After an email bounces, it goes to the cleaned portion of your audience. If the cleaned email address is still valid and the contact wants to resubscribe to your audience, they need to sign up through your signup form to opt back in.

In this article, you'll learn common reasons a valid email address could bounce.

Reasons valid addresses bounce

Denied by Firewall or Filter

If your email was blocked, a spam filter may have interpreted the campaign content as spam. Spam filters can be on the recipient's computer, on their company's email firewall, at their ISP/Data center, or all of the above. Common SMTP replies for spam filtered emails include Denied, Unacceptable Content, Blocked, and similar messages.

If the email address is someone you know, use the Send a Test email from Mailchimp—just send a quick, plain-text email to a single recipient. If a "Hi, how's it going?" type of personal message gets through, but your normal email campaign doesn't, it's likely an issue with the campaign's content.

Read our guide on how spam filters think to see if anything mentioned there sticks out about your content.

Is Mailchimp blocked?

Corporate spam filters and email firewalls can be very strict and treat a large amount of identical incoming emails as spam. If the spam filter or firewall is used to seeing you send one or two personal emails to a client in the company, they may interpret a bulk send made on your behalf as spam.

A block can be temporary if the recipient email server thinks too much email is coming from one server too fast. Other times the block is permanent and the only way to get around that is to ask the IT group in charge to whitelist Mailchimp IP addresses.

Is a major ISP blocking your emails?

If a major ISP like Gmail or MSN/Hotmail is blocking your emails, this is typically only temporary. ISPs usually unblock senders within a day or two after spam complaints subside.

If your From email address is at AOL or Yahoo and you see a high number of bounces, it is likely that the delivery of your campaign was affected by changes in DMARC policies.

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