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.

Beginning February 2024, Gmail and Yahoo will require a custom authentication and a published Domain-based Message Authentication, Reporting & Conformance (DMARC) record for anyone sending more than 5,000 emails to Gmail or Yahoo addresses in a 24-hour period. To prevent your emails from bouncing, we strongly recommend authenticating your email domain and configuring DMARC.

Also, if you use a free email service like Gmail or Yahoo for your From email address, we strongly recommend you switch to an email address from a private domain, like the one you use for work or for your website.

For more information about custom authentication and DMARC, check out About Email Domain Authentication.

Reasons valid addresses bounce

Denied by Firewall or Filter

If your email was blocked, a spam filter may have interpreted the content as spam. Spam filters run on the servers receiving and processing your subscribed contacts' emails. 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 to 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 doesn't, it's likely an issue with its 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 can be very strict and treat a large amount of identical incoming emails as spam. If the spam filter is used to seeing you send 1 or 2 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's 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 allowlist Mailchimp IP addresses (other providers may refer to it as "whitelist").

Is a major ISP blocking your emails?

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