What could cause temporary bounces due to user does not exist errors?
Summary
What email marketers say9Marketer opinions
Email marketer from Email Geeks suggests that online.de might have changed their MX records to point to Apple iCloud servers for a few hours.
Email marketer from Quora suggests that aggressive anti-spam filters at the recipient's end could temporarily reject emails, falsely reporting the user as non-existent. This can happen if the email triggers specific spam filters.
Email marketer from Mailjet explains that recipient mail server downtime can lead to temporary 'user does not exist' errors. If the recipient server is temporarily unavailable, the sending server may report the user as non-existent until the recipient server is back online.
Email marketer from Neil Patel's Blog explains that typos in email addresses or the recipient closing their email account are common causes for user does not exist errors, which can be temporary if the user later corrects the typo or reactivates the account.
Email marketer from Reddit shares that temporary DNS resolution problems on the sender's side can also cause these bounces. If the sender's mail server can't resolve the recipient domain's MX records, it might incorrectly report the user as non-existent.
Email marketer from Litmus explains that there may be internal DNS server issues at the ISP, especially after a DNS change, can cause 'user does not exist' errors. They suggest working with IT to check that DNS settings are correct.
Email marketer from Stackoverflow mentions that greylisting, a spam prevention technique, might cause temporary bounces. The first email attempt might bounce, but a legitimate server will retry, and the email will then be delivered.
Email marketer from SuperUser forum shared that one possibility could be a server migration or other major change on the recipient's end, where user accounts are temporarily unavailable or misconfigured.
Email marketer from EmailDeliveryGuru.com shares that temporary MX record issues or DNS propagation delays after a change can cause 'user does not exist' bounces. If the MX records are temporarily misconfigured or not fully propagated, emails can be misrouted.
What the experts say5Expert opinions
Expert from Email Geeks shares that Farsight sees no change for Kundenserver.de since 2014.
Expert from Email Geeks confirms that Farsight saw the Apple bounce issue, supporting the theory of a temporary MX record change.
Expert from Email Geeks provides a link to SecurityTrails, suggesting it might indicate that the issue isn't a simple DNS problem.
Expert from Word to the Wise indicates that DNS propagation issues could cause temporary bounces, especially if the recipient domain recently changed its DNS records. During propagation, some servers might still point to old records, leading to 'user does not exist' errors until the DNS updates fully.
Expert from SpamResource.com explains that temporary bounces, including those indicating 'user does not exist,' can be caused by greylisting techniques employed by recipient mail servers. These servers temporarily reject emails from unknown senders to combat spam, causing an initial bounce. Legitimate servers retry, and the email is eventually delivered.
What the documentation says5Technical articles
Documentation from Microsoft Support explains that a 550 5.1.1 error indicates an invalid recipient. This could be due to the user not existing at the domain or a temporary issue preventing mail delivery. The problem might resolve itself, or it could require the sender to verify the recipient's address.
Documentation from Sendgrid shares that their system might temporarily block an email if it detects suspicious activity. This could result in a bounce message indicating the user does not exist, even if the email address is valid. These blocks are typically lifted automatically.
Documentation from Google details that Gmail might return a temporary failure if it's experiencing high volume or detecting unusual sending patterns. This can result in a temporary bounce message indicating that the user does not exist.
Documentation from RFC Editor explains that 5.1.1 'Bad destination mailbox address' may be caused when the mailbox name is incorrectly formed, or the mailbox simply does not exist. This error may be permanent or transient.
Documentation from AWS details that with SES, a 'user does not exist' bounce often means that the recipient's email address is invalid or that there's a problem with the recipient's mail server, which can be transient. They suggest reviewing bounce codes for details.