Understanding Unknown Results

When validating email addresses, you may see some results come back with an Unknown status. This means Truelist was unable to definitively determine whether the email address is deliverable or not. This article explains why unknown results happen and what you can do to minimize them.

Why do unknown results happen?

Unknown results occur when Truelist cannot get a clear answer from the recipient’s email server. There are several reasons this can happen:

Server timeouts

The most common cause of unknown results is a timeout. When Truelist connects to a mail server to verify an address, the server may be slow to respond or may drop the connection entirely. If the server doesn’t respond within the allotted time, the result is marked as unknown.

Greylisting

Some email servers use a technique called greylisting as a spam prevention measure. When a new sender tries to deliver a message, the server temporarily rejects it and expects the sender to retry after a delay. Legitimate mail servers will retry, while many spam bots won’t.

During email validation, greylisting causes the initial verification attempt to be rejected. If Truelist doesn’t wait long enough to retry, the result comes back as unknown.

Rate limiting

Email servers often limit how many verification requests they’ll accept in a given time window. When you’re validating a large batch of emails that are heavily concentrated on a single domain (e.g., thousands of @gmail.com addresses), the recipient server may start throttling or blocking requests after a certain point, causing the remaining emails to come back as unknown.

Temporary server issues

Sometimes the recipient’s mail server is simply down, undergoing maintenance, or experiencing issues. In these cases, no amount of retrying will produce a result until the server comes back online.

How to reduce unknown results

Use the Thorough validation strategy

The most effective way to reduce unknown results is to use the Thorough validation strategy. The Thorough strategy is specifically designed to minimize unknowns by:

• Waiting longer for greylisted servers. Instead of giving up quickly, Thorough uses 5-minute retry delays, giving greylisted servers time to accept the retry attempt.
• Retrying when rate limited. When a mail server is temporarily throttling requests, Thorough will wait for cooldowns to expire and retry rather than immediately giving up — retrying multiple times over approximately 12 minutes.
• Producing more definitive results. The tradeoff is that Thorough takes longer to complete, but it will turn many would-be unknowns into definitive ok or invalid results.

Spread your list across multiple domains

If your list is heavily concentrated on a single domain (e.g., mostly @gmail.com or @outlook.com addresses), the recipient mail server is more likely to rate-limit validation requests. Where possible, try to validate lists that contain a mix of domains rather than batching all addresses for one domain together. Spreading your emails across diverse domains reduces the load on any single mail server and leads to fewer unknowns.

Re-validate unknown results

If you’ve already run a batch and have unknown results, you can download just the unknown emails and re-validate them in a new batch. This is especially effective if:

• The original batch was large and caused rate limiting
• The recipient server was temporarily down during the original validation
• You used the Fast or Accurate strategy originally and want to try Thorough

Validate at off-peak times

Email servers are more likely to be responsive and less likely to throttle during off-peak hours. If you’re consistently seeing high unknown rates for certain domains, try running your validation during evenings or weekends (relative to the recipient server’s timezone).

What unknown rate should I expect?

For most lists, you should expect an unknown rate of 2-5% using the Accurate (default) strategy. Some factors that increase the unknown rate:

• Large batches to a single domain — more emails to the same server means more chance of rate limiting
• Strict mail servers — some providers are more aggressive with greylisting and throttling
• Older or less maintained lists — domains may have changed mail servers or gone offline

Using the Thorough strategy, you can typically bring the unknown rate down to under 1% for most lists.

Choosing the right strategy for your needs

For more details on each strategy, see Validation Strategies.