Email Address Verification: Why Some Tests Produce Unknown Results

email address verification

As part of Markitors email list management services, we conduct email address verification using a variety of bulk email verification tests.

Our email address verification tests perform a deep cleansing SMTP test that connects to a recipient’s mail server, asks if the mail box exists and then disconnects right after receiving the reply.

We remove duplicate email addresses, invalid syntax, invalid domain dns, invalid mx dns, disposable addresses, role accounts, spam traps, honey pots, blacklists, catchalls, greylisting, mailbox full and mailbox does not exist. Basically, any email address that may bounce and hurt your sender score.

We want your email campaigns going to inboxes instead of spam boxes.

Here are a few of the email address verification tests we run:

1. Addresses are verified for being in a valid RFC 2822 compliant format.

2. All addresses are filtered against each other to remove any duplicates.

3. Addresses are filtered against the list of disposable email address providers and will be failed if found.

4. Addresses are filtered against your blacklists and if found are failed.

5. After an address has passed all other tests, we perform a deep cleaning SMTP test to verify the mailbox actually exists on the receiving mail server.

We have 3 main types of mail servers that will reply as “Unknown”:

1. Catch-all – Catch-all’s are mail servers that will accept mail regardless of whether the mailbox exists or not, rendering the SMTP test useless.

2. Greylisted – Greylisting is a method to prevent spammers from sending emails by adding an automatic delay and is not compatible with the SMTP test.

3. Transient – Transient errors consist of issues in connectivity that are temporary and can be a result of a high mail server load, slow to respond which ends in a timeout and disconnects.

The issue with these unknown email addresses is actually in how the receiving mail server is configured that makes the test inaccurate.

It is 100% unknown if these addresses exist or not. Emails are flagged as unknown when the system can not perform a SMTP test on the mail server. If you sent 100 emails to unknown addresses it could result in a 100% delivered campaign or a 100% bounced campaign. Unknown addresses will not consume any validations from your account.

We recommend to only send emails to the passed addresses however if you choose to send to the unknown addresses as well, we advise adding small batches of the unknown addresses while sending emails to the passed addresses and then simply manually handling any bounce events that may occur.

You can indeed reprocess your Unknown lists over and over in the attempt to pull out more passed/failed addresses. However, not all types of Unknowns are able to be processed.

Catch-all – Catch-all’s will always return the same exact Unknown result.

Greylisted – Greylisting addresses can possibly respond with a different result when retested.

Transient – Transient results (Details = “None.”) are temporary errors which mean a passed/failed result is possible when retested.

After many times of reprocessing you will experience diminishing returns in which you will no longer retrieve more passed/failed addresses from the list.

Markitors is a MailChimp Expert full service email marketing agency that handles email newsletter management, email marketing campaign strategy, email marketing automation, and email marketing audits for small businesses.

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