If you are an e-mail sender with a spotty or poor reputation, you should know by now that the big ISPs are more likely to block your e-mail or to deliver it to subscribers’ bulk folders.
Reputation is the key driver of inbox deliverability. A Return Path study found 77 percent of e-mail was blocked at the ISP level because of sender reputation problems, compared with 17 percent for content issues, and 6 percent for dubious URLs.
But it can be hard to know what your reputation is. Aside from Return Path’s proprietary Sender Score ranking service, reputation is hard to quantify.
You can get some idea of your reputation, as ISPs view it, by looking at your delivery reports and complaint volumes by domain instead of in the aggregate. Also, monitor inbox versus bulk-folder delivery with test e-mail accounts or a third-party service.
Delivery Management Tactic: Throttling
Besides blocking and filtering, many ISPs now use throttling to manage e-mail from senders with dubious reputations. Throttling is a process that admits only a limited number of messages from a particular sender within a set period and blocks all messages above that limit.
Throttling itself isn’t a new practice. Many ISPs use it to manage message delivery from high-volume senders or questionable sources. Now, however, I’m seeing throttling used because of the sender’s reputation as well.
How to Tell If Your E-mail Was Throttled
Although throttling is a little more forgiving than an outright block, it’s harder to detect; thus, its effects aren’t as obvious.
When an ISP blocks your e-mail, it usually sends a bounce message reporting the delivery failure. But you would know something was wrong even without a bounce message, because those blocked messages would generate absolutely no opens or clicks.
If an ISP is throttling your messages, however, you’ll still see open and click activity on the percentage of messages that were delivered. Also, each ISP has its own standards for throttling.
This means you have to pay even greater attention to your positive and negative performance metrics — at the domain level. A block due to throttling at a major ISP might show up as only a decrease in overall performance, but it would be very apparent at the domain level.
Here’s a real-life example: One sender’s average bounce rate jumped from 2.5 percent to 7 percent on a weekly sending volume of 2.2 million. This increase was due to throttling. Most of the bounces were reported back as soft bounces, meaning the addresses were legitimate and could be retried later.
Meanwhile, the spam-complaint volume remained consistent, about 0.35 percent. This was misleading, however, because a large percentage of intended recipients never saw the e-mail. So the number of potential complainers was reduced.
Open and click-through rates declined marginally, because the divisor is the number delivered, not the number sent. You wouldn’t expect to see a major decline in these numbers.
Hotmail’s Response to a Throttled Sender
Consider Hotmail’s response to one sender:
- Your IP (xx.xx.xxx.xxx) has been intermittently blocked as the server(s) connecting to Windows Live Mail/Hotmail has exceeded our daily/hourly limit thresholds.
Hotmail/Windows Live Mail limits the number of e-mail messages a particular IP can send within a time period. Based on an IP’s reputation (built based on various data sources) it is allotted an allowed sending limit per unit of time. When an IP (sender) exceeds its allowed limit, any further SMTP commands from the IP will receive the SMTP error code 421 from Hotmail/Windows Live Mail and the connection terminated.
Here’s the kicker: the reputation “secret sauce” is hidden. When you attempt an unblock request, the ISP might reevaluate your reputation. Sounds like a good deal, assuming you move the needle in the right direction. However, this reevaluation might also lower your limit. Or, the ISP could conclude it was correct to limit you and leave your limit as is.
The lesson: no resolution attempt guarantees a successful outcome.
Managing and Improving Your Reputation
These issues help shape your reputation:
- Your domain or IP address
- Recipient feedback (spam or fraud complaints)
- Open and click action or inaction
- Traffic patterns (sending to invalid addresses, spam traps)
- List hygiene
- Longevity of your domain or IP
- How you process unsubscribes
- Use of authentication
- Use of third-party reputation agencies
- How you follow e-mail protocols
- Your affiliate and partnership relationships
- Reports from anti-spam agencies
- Behavior of other senders in your IP neighborhood
If you’re pretty sure your sender reputation could use some improvement, try these strategies:
- Police it at all times by monitoring your delivery reports, establishing feedback loops with ISPs that offer them, and watching major blacklists like Spamhaus, which report data to the ISPs.
- Chart delivery, open, and click rates over time to identify problems before you sustain a complete block at a key ISP.
- Switch to double opt-in or take other measures to reduce the number of undeliverable addresses.
- Process unsubscribes immediately.
- Remove addresses associated with spam complaints as soon as an ISP reports them.
- Long term, implement strategies to reduce complaints (matching spam complaints to frequency or content changes, managing subscriber expectations, etc.).
You can improve your reputation by following e-mail best practices for list acquisition and hygiene. Are you ready to do it?
Until next time, keep on deliverin’!
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