Why am I getting a 550 Sender Not Verified error for a small percentage of recipients with a correct return path?
Summary
What email marketers say10Marketer opinions
Email marketer from Campaign Monitor explains that A bounce rate is the percentage of total emails sent that couldn't be delivered to recipients' inboxes. There are two types of bounces—soft bounces and hard bounces.
Email marketer from GMass suggests that even with correct configurations, sender reputation can influence deliverability. A new IP address or domain might face stricter scrutiny initially, leading to temporary blocks.
Email marketer from Email Geeks shares the cause was a misconfigured server worker that stopped restarting from config changes and the return path being a subdomain was a red herring.
Email marketer from EmailOctopus suggests that a '550 Sender Not Verified' error could arise if the recipient's server is unable to verify the sender's domain. This may be due to missing or incorrect DNS records.
Email marketer from Neil Patel Digital explains that a 550 error can indicate an issue with email authentication (SPF, DKIM, DMARC). Even if set up, there could be misconfigurations causing sporadic failures.
Email marketer from Klenty responds Error message 550 Sender Verify Failed means that the destination server was unable to successfully verify the email sender's authenticity. This could be because of the wrong SPF record, domain key, or any other authentication failure.
Email marketer from StackOverflow responds that it can be caused when the return-path domain has a problem. Perhaps the domain is too new, or has a low reputation due to lack of history. This can look suspicious to the recipient server.
Email marketer from Sendgrid explains A 5xx SMTP response code means a permanent error has occurred and that your message could not be delivered. It’s important to note that a 5xx bounce is permanent: the email address is invalid, the email address does not exist, or the recipient’s email server has blocked the email.
Email marketer from Reddit shares that a temporary 550 error on a small percentage could be due to temporary DNS propagation issues after making changes to SPF/DKIM records.
Email marketer from Email Marketing Forum details if you're on a shared IP, another user's actions could negatively impact your deliverability, causing temporary 550 errors.
What the experts say3Expert opinions
Expert from Spamresource details, Error 550 is a somewhat generic SMTP error which usually indicates a policy rejection of the message.
Expert from Word to the Wise explains the 550 error can indicate a problem with SPF/DKIM setup, poor sending reputation, or the recipient server's policies. It is vital to analyze bounce messages and check sending infrastructure configurations.
Expert from Email Geeks suggests the issue might be related to unresolvable RFC 5321 domain at Yahoo and that with a tiny fraction of rejections, it could be a minor MBE issue or a random error, also asks if the rejecting recipient domains are similar.
What the documentation says4Technical articles
Documentation from SparkPost explains that 5xx errors indicate permanent failures. A 550 error specifically suggests the sending address was refused. They recommend checking the sending domain’s reputation and authentication setup.
Documentation from Microsoft details that a 550 5.7.1 error often means the sender is blocked because they are unauthenticated. It suggests verifying SPF and DKIM records are correctly configured for the sending domain.
Documentation from RFC details The enhanced mail system status codes are intended to provide more structured and reliable information than the textual explanations accompanying the basic SMTP status codes
Documentation from Mailjet explains that the 550 SMTP error often indicates the email was rejected because the mailbox is unavailable or the recipient server suspects spam. This may be caused by policy reasons or authentication failures.