We often get a question asking why they see “unknown” status listed next to an email address which has been verified. While the causes differs time to time, here are a couple of answers that would explain further and hopefully help you have a better understanding of the results we provide our customers.
What are "unknown" status codes?
“Unknown” is one of the outcomes that our software generates as a result of it falling outside the categories of valid, invalid, undeliverable, unreachable, disposable, and others. When our software concludes an email address as being “unknown”, it would mean that we tried every single trick under our sleeve to validate that email address. Some of these tricks include verifying the syntax, domain checks, mta checks, and many others. You can see more about what is checked on our email verification process page. After thorough checking, we were unable to conclude that email address being valid or invalid or whether its deliverable or not.
When do we put “unknown” next to an email address?
The most common reason for putting this status code is when the mailserver that we are trying to connect to doesn’t reply back with yes or no. Usually when we verify email addresses, mail servers usually reject mail if the user is not known or doesn't exist. It also throws out a whole lot of other errors for other reasons like Mailbox being full and so on. So when the mail server doesn't respond back with a definite yes or no, we tend to report back in similar manner. We mark it as “unknown”.
How can you identify an email address as being “Unknown”?
Most of these mail servers don't respond back because they only give you the status of the email address AFTER you finish sending the message. This means we have to send a message to your recipient before we get a bounce message. For us, it defies the purpose of using our service since you could easily send a message yourself and figure out whether it exists or not. We do have real-time bounce processing available which we aim to use for sending verification emails to your customers. This is optional and is still in beta stage. At this current stage, We DO NOT send an email to any of your recipients.
What other types of events can cause the “Unknown” status?
Another common reason a result is marked as being “Unknown” is when it times out. We have timeout script in place which basically gives up on a connection if its not responding for a certain amount of time. This happens more so during API interactions than direct upload. After a couple of tries, if its still timing out, we mark the email address with the status as “Unknown”. In simple of terms, we want to talk to mailserver but the mailserver is not talking back to us so we say…that’s ok…we will come back later to say Hi again.
How many “unknowns” will there likely be in my list?
Its really hard to predict a number as each list has varying domains and email addresses. For B2B lists, we observed a rate of anywhere from 10% up to 60% and 15%-25% being the average. Most of the email addresses that are US based tend to say “Hi” relatively fast and respond back with status on whether an email address exists or not. The issue is with International Mail Service Providers who don't like talking a lot.
In conclusion, the best way to put it in terms of “unknown” status is being neither good or bad. Its not something to avoid sending to but neither should you have 100% confidence that it won't bounce. Our suggestion is to split the unknowns into separate file and include them with a greater number of valids. That way, over time you will know easily which ones are valid and which ones are bounces. This will also help you keep your reputation up as a sender.
Another suggestion is to submit "unknowns" over time so our system can validate them again. Sometimes, ESPs allow verification of those IPs later in time that they once didn't allow. This could reduce the number of unknowns in your list and help you in achieving better deliverability.