What causes hard bounces?
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A hard bounce tells you something simple but important: that email address isn't going to work. The server at the receiving end sent back a permanent failure, usually an SMTP 550 error, which translates roughly to "this address doesn't exist here."
The most common causes: the address was mistyped at signup (captain@deepcurrent.io entered as captian@deepcurrent.io), the domain has expired or shut down entirely, or the mailbox was deleted after the person left the company or closed their account. In all these cases, retrying won't help. There's no mailbox there to receive it.
The other big cause is poor data collection. Purchased lists and scraped addresses are full of hard bounces, which is one of the many reasons you shouldn't be mailing to them in the first place. Even organically collected lists accumulate invalid addresses over time as people change jobs and providers.
Hard bounces should be removed from your list immediately and permanently. Most ESPs do this automatically. If yours doesn't, set up a process. Continuing to mail hard-bounced addresses signals to mailbox providers that you don't maintain your list, which hurts your bounce rate and your sender reputation. If you want to catch invalid addresses before they bounce, a list validation tool can flag bad addresses at the point of signup or before a send. That's something Review My Emails does as part of RME Clean.
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