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Seek and Destroy Invalid Addresses to Avoid Temporary Blocking

  |  February 24, 2010   |  Comments

How to ensure your e-mail marketing messages will get cleared by ISPs.

Even e-mail marketers who practice regular list hygiene can find themselves unable to mail to a specific ISP because a campaign generated a temporary block at an ISP. The bounce code reporting the block usually says the sender generated "too many invalid e-mail addresses."

This occurs more regularly because many marketers are trying risky acquisition tactics to grow e-mail lists quickly. Quality still outweighs quantity in the long run. Legitimate marketers should understand the trade-offs.

If you engage in poor list-building techniques, you might lose the privilege to market to all your subscribers, not just the ones you added quickly.

A temporary block due to invalid e-mail addresses isn't as serious as a permanent block, because it usually lasts 24 to 72 hours and doesn't require you to contact the ISP and ask to have the block removed in most cases.

However, you won't be able to send to your valid addresses or identify the invalid addresses to clean out your list until the block lifts. And, you could face stricter or even total blocking at that ISP if you don't resolve your invalid-address problems.

How Invalid E-mails Affect Deliverability

Sender reputation is the most important factor affecting your ability to deliver to the inbox at most ISPs. Having too many invalid addresses is one of the top three problems that can hurt your sender reputation. (Too many spam complaints and low engagement metrics are the others.)

Invalid addresses are a red flag to ISPs, because it's a common spammer tactic to generate long lists of random e-mail addresses and send messages to see which ones actually get delivered. This is also called a dictionary harvest attack.

When an ISP determines that a certain percentage of inbound e-mail from the same sender contains a high number of invalid addresses, it imposes a temporary block. This generates a bounce message saying blocking is due to too many invalid e-mails.

The rates that trigger such blocks vary. ISPs don't disclose their thresholds in order to prevent spammers from varying their list sizes to get around the blocks.

Withholding this data from senders is an ISP security measure. Sadly, even spammers read the blogs, complain to postmasters, and read bounce messages in an effort to defeat the ISP's measures to protect its customers, who also are your subscribers.

Am I a Target for Temporary Blocking?

If you practice good list hygiene and remove invalid e-mail addresses after the first bounce, you likely won't have a problem with temporary bounces.

Three categories of e-mail senders are more vulnerable to temporary blocking because of invalid addresses:

  • Those who recently changed e-mail service providers and didn't practice effective or regular list hygiene.

  • Those who periodically go back to old data and try one more time to revive non-responders.

  • Those who use e-mail address appending to build their lists without rigorous attention to quality.

See my previous ClickZ columns for more advice on using appending and avoiding its dangers.

Minimize Your Exposure to Blocking for Invalid Addresses

  • Clean up e-mail address collection. This is the first and best place to start. Switching to double opt-in, or adding an e-mail confirmation to single opt-in can trap and remove bad addresses before they become a permanent part of your database.

    You also need reliable offline collection procedures to minimize misspellings or falsifications. I offer four tips to improve offline acquisition in this column.

  • Remove addresses from old files. This is more difficult, requiring time and patience, but you must do it if you are finding your messages blocked because of too many invalid addresses.

You have two ways to clean up this older data:

  1. Isolate the ISP: First, identify the ISP that blocked your e-mail. Try to calculate how many good addresses were delivered before the block was imposed. This number is your estimated daily limit.

    Schedule a "timeless" campaign (one that doesn't have an immediate expiration date) using that daily limit as a guide until you have worked through all the names on your list.

    In order to make this work, you must capture and remove invalid addresses as they come through to avoid sending to them a second time. Repeatedly e-mailing invalid addresses is another spammer tactic and ISP red flag.

    Besides, it's a waste of resources, both on your end, and the ISP's end, to constantly report what you should already know - that the address isn't valid.

  2. Split your file: Divide your mailing list into two files, one with addresses that have been delivered and one with addresses that were blocked.

    Send to your "delivered" file first, ensuring that these messages do get delivered. Then, start sending to the addresses in your "blocked" file, a few at a time, until a block occurs. Every time you mail to the list after that, you must front-load your good addresses so that they're ahead of the block.

    Continue this process until you have made the way through all of your addresses for that ISP.

Patience Is a Virtue

Both of these tactics require slow and steady work. You might need several days, or longer, to work through the process, depending on how many of that ISP's addresses you have on your list.

Patience is the only true solution. However, your efforts to clean out invalid addresses time will be wasted unless you tighten up your address collection at opt-in, improve offline acquisition, and avoid risky acquisition methods.

Until next time, keep on deliverin'!


Stefan Pollard

Stefan Pollard, who started his career in online marketing in 1999, was considered a selfless mentor and champion of best practices in e-mail marketing. He held the position of senior strategic consultant at Responsys where he was responsible for developing e-mail marketing and lifecycle messaging strategies to increase clients' ROI. Before that, Stefan led the e-mail consulting program for Lyris clients, frequently speaking at industry events on best practices. Prior to that, he managed the audit process and consulted with clients to improve their e-mail delivery challenges for Habeas. As an e-mail marketer, he spent several years building and executing acquisition and retention campaigns at E-Loan and Cybergold.com. He died May 14, 2010.

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