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Email Error MessagesIf you’re in sales — make that any profession in the modern age — you’ve likely struggled with deciphering what email error messages actually mean. While email has opened a world of direct access to key decision-makers, the medium is not without its challenges.

Did it bounce? Is the recipient’s email inbox full, and thus not receiving new messages? Or worse, has someone in your organization not played by the rules, and gotten your whole company domain blacklisted from their server? What does this cryptic, puzzling email error message mean!?

Luckily we have an entire research team whose job it is to collect, verify and update contact information. Having seen virtually every email error message that exists along the way, they’re uniquely versed in the language of email deliverability. Relying on our team’s expertise, we’ve put together this guide for common error messages, complete with solutions, so that next time you’re faced with the question “what does this email error message mean?” you can find what you need to diagnose and remedy the problem.


Error message example: <>: host said: 550 <>… User unknown

The problem: The intended recipient is not recognized by the server, indicating they’re either no longer with the organization, or that you’ve spelled something wrong. This is probably the most common error message, and perhaps the most misunderstood, so if you come across it, keep reading.

The solution: The obvious first step is to make sure that you have typed in the email address properly, as the vast majority of the time, this error means that while the domain (the text after the @ symbol) is valid, the specific user (the part before the @ symbol), is not. Make sure you spelled everything correctly, and that the recipient is still with the company.

While that accounts for the vast majority of instances, in a very small fraction of cases, it’s possible that the recipient organization’s ISP is somehow causing a valid user’s email to bounce. While rare, if you’ve confirmed that the email address is in fact correct, you should contact the user by other means (picking up the phone is a great start) and let them know that an email you’ve been told is valid is registering a bounce. It may sound complicated, but if you help a person fix this problem, they might feel the least they can do is take a meeting with you!

#PROTIP: Everyone gets an email address wrong now and then, but sending the same server an excessive amount of emails that trigger “unknown user” error messages could land you in trouble, and by trouble, we mean someone’s junk folder. First, your emails are relegated there, and if you continue to send emails that trigger 550 user unknown error messages, the receiving system might stop attempting to deliver your emails altogether, or even blacklist your domain. To stay on the right side of the email gods, immediately remove emails that trigger this error from any lists you use, and have your email marketing manager promptly remove them from any marketing email lists as well.


Error message example: <>: User is over the quota. You can try again later.

The problem: The recipient’s inbox is so full that new messages can no longer get through.

The solution: Unfortunately, there is not too much you can do on your end to get through to this particular inbox. However, this is often an indication that the recipient is no longer checking this email address, so you should get yourself a new contact. Perhaps they’ve moved to a different division of their company that uses a different email format, or have left the organization altogether. At one point in time, they may have set up an auto-reply to tell people who email this address that they no longer use it, but because the organization no longer monitors and clears out the inbox, new messages cannot get through, and so the auto-reply that would give you more information, like their replacement’s email address, never goes out.


Error: Connection Time Out (Sometimes: Connection Refused)

The problem: This error occurs when a high volume of incoming messages have triggered the email equivalent of a busy signal. This server’s auto shutdown is meant to protect against things like cyber attacks, or what IT teams call “mailbombs,” whereby tons of emails are sent to a particular domain in order to hack or disable it through malicious code. In order to protect the server, organizational security, and individual users, the server will literally stop accepting messages until either the high volume stops, or sometimes, until a system administrator has diagnosed the problem and gives the system the green light to resume.

The solution: Like receiving a busy signal on the phone, your best bet is to try back later. That does not mean try ten times in the next ten minutes — if you do, you could actually be contributing to the organization’s time-out problem, and in the worst case scenario, could be mistakenly identified as a malicious sender. So be patient, wait an hour and try again. If you receive the same message, wait 24 hours. If you still receive the same message, try calling the organization, as its very rare that a legitimate company would allow a connection time out to halt operations for a whole day, so there could be something else at work.


Error Message: <>: host refused to talk to me: 550 Permission denied

Alternate versions of this message may contain additional text that’s, well, a little less cryptic: <>: connect to 550 Connection refused — stop spamming!

The problem: Your sender’s name (domain), or even your specific email address, has triggered a spam filter.

The solution: First, take a deep breath. Resist the urge to publicly shame that one email-happy co-worker, or the marketing team. Your domain can actually become blacklisted accidentally, so while proper email etiquette education may be in order, it’s possible no one person is to blame.

To remedy the situation, you’ll need to contact the company’s email systems administrator. Provided that you are not actually a spammer, most of the time, the administrator will be happy to whitelist you (allowing you to send emails through). Understand that with more and more actual spam going around these days, administrators are forced to adopt and enforce stricter blacklisting procedures, and as an unfortunate consequence, there are times when valid senders are caught in the net. Rest assured that as long as your objectives are legitimate, most email administrators are happy to whitelist you, and are actually count on legitimate senders to let them know that their ISP settings may be blocking emails that are not spam.

To ward against this happening in the future, we also recommend contacting your own email administrator, sharing the error message, and having them share with the broader company best practices on emailing. Even if your domain was blacklisted accidentally, it doesn’t hurt to remind everyone to be cool and place nice so that you can continue to communicate freely and avoid being locked out.

Looking for a reliable source of email addresses for sales prospects? Request a free trial of Winmo, the evolution of The List Online, to find what you need. Interested in data hygiene, and in need of giving your own CRM a clean bill of health? We can help with that too.