Common Issues Encountered Verifying Email Addresses

Verifying Email Addresses is a complicated business.

You would not be here if that was not the case. Hard bounces are a set metric for ESP’s and delivery. Too many hard bounces and your delivery tanks. Over five to ten percent and most ESP’s will kick you off the platform. Many domains these days use them as a way to block senders and gauge whether the mail is good or not. uses multiple methods to verify the email addresses for hard bounce removal. We use static tables, MX lookups, and a final SMTP connection to provide the most accurate reporting possible.

Many of the large service providers like Gmail, Hotmail, and AOL provide bounce replies in real-time during the SMTP session. Some email providers do not give real-time bounce replies but will send the bounce messages via email. Some providers will accept all messages to all recipients whether they are valid or not and do not provide replies. This makes it impossible to accurately detect if an email address is valid or not until you actually send an email to the recipient. Yahoo is an example of one such provider.

Many mailservers do not cooperate or provide a false positive on purpose. This may be an due to a difficult admin or can be used as a form of spam management. With these domains it is difficult to determine if the email address is actually valid or not.

Since some ESP’s do not cooperate with certain checks a final SMTP connection is recommended for best results. To further enhance your data and emails which took action are reported back with a “responsive” tag. This not only allows you to remove the bottom percentage of BAD data, but identify the top portion of GOOD data.

When our system is unable to conclusively verify the status of an email address, it it marked as “unknown”. These emails may or may not be valid. Examples of this status can be catch-all email addresses, certain soft bounce return codes, or temporarily unavailable email addresses.

At we aim to provide the highest level of email verification possible, but at times circumstances are out of our control. The following situations are possible issues we can encounter while verifying email addresses.



False Positives with Email Validation

In certain cases email addresses may be marked valid and may in fact be invalid. This false positive can be caused by several reasons. A rogue mail admin may wish to monitor traffic on his network to block users. Some providers (free mail providers many times) may do this as a part of their anti spam protection. In other cases invalid emails are reactivated so administrators can use them as honeypots for spammers. And finally some systems identify any email in their database as active once setup, and do not reply with the correct bounce message until you actually send a campaign.


Temporary SMTP Server Issues

The majority of the unknown results from our email validation system are caused by temporary issues. These may include the following reasons:

  • SMTP server timeout or downtime
  • Greylisting
  • Mailbox size exceeds quota
  • Temporary mailbox suspension
  • Temporary blocking due to low IP reputation

Due to these reasons it is strongly recommend to verify the unknown status of the list down the road. We do not recommend deleting the unknown potion of your list immediately after verification. Chances are that previous emails which previously tested as unknown are in fact valid.



Greylisting is when a mailserver returns a temporary failure response or “temporarily unavailable” on the first attempt to deliver an email. It may accept it on the second attempt. A correctly configured email server will attempt to redeliver a message after a temporary failure response x number of times depending on it’s configuration.

Since greylisting is a temporary issue from the remote mailserver we do not recommend deleting the unknown results due to greylisting. These “unknown” status emails may in fact be valid as previously mentioned.


Catch-all Email Domains

Our email validation service has the ability to detect catch-all emails. A catch-all email account is an address that is specified to receive all messages that are addressed to an incorrect email address for a domain. These emails may be “valid” or “invalid” but the mailserver accepts fake, non-existent, and valid email addresses. In some cases, these catch-all domains are used by ISPs and ESPs as spam traps and for spammers using dictionary spam attacks.

This makes it very difficult to verify if the email is valid or not on our end until a bounce reply is received. Our final SMTP feature helps in this regard or you can mail it on your end.


Email Service Providers Not Following Protocol

Some ESP’s do not follow normal protocol and have certain features turned off by default. This is done to protect their network. A real email must be sent in order to trigger a response. This makes it harder to verify if the email is valid or not. We suggest a real campaign deployment after verification to remove any last bounces you may encounter.