Summertime and the Blacklisting Is Easy: Tips to Steer Clear in the High Season

Summer is high season for email blacklisting. What can marketers do to avoid being put on the dreaded blacklist, which could effectively ruin their email ROI?

Like most people, when I think about summertime I have images of long days, abundant sunshine, outdoor activities, weekends spent on the beach, barbecues, and ice cream running through my head. However, if you're an email marketer, you might want to think about blacklists.

Why? This is the second-highest time of year, after the November-December holiday season, for blacklisting. And if you're an email marketer in Brazil (where it's not technically summer), getting blacklisted shouldn't just be on your mind – it should be top of mind. According to Return Path's research, Brazilian senders get blacklisted the most, with 79 percent of IP addresses being listed at least once.

Even if you don't know much about blacklists, you can probably tell from the term alone that it's something bad. And it is. A blacklist is a list of domains and/or IP addresses that have been reported to be "known" sources of spam. They are available to the public (mostly for free) and exist to help mailbox providers protect their users from unwanted email. If your practices and your content look spammy, you could wind up being blacklisted regardless of whether or not you're a legitimate, permission-based sender.

While there are hundreds of well-known public blacklists, not all are created equal. Being blacklisted on some could have a negligible impact on inbox placement, while others could effectively decimate email return on investment (ROI) for your program as long as your IPs or domains remain on the backlist. Prominent IP-based blacklists include Return Path's Reputation Network Blacklist, Spam Cop,,,,,, and Domain-based blacklists include, URIBL, and SURBL.

Just how damaging is being blacklisted for email program performance? Our research shows that inbox placement rates at Gmail, which correlate strongly with Spamhaus and CBL blacklisting events, decline by up to 58 percent. When you consider that typical Spamhaus blacklistings can last up to eight days and a typical CBL blacklisting can last up to 14 days, it can have a significant impact on your ability to drive opens, clicks, conversions, and revenue from email. The day of the week you send email could also have an impact. Wednesday is the most common day to be blacklisted at Spamhaus, with 68 percent of listings occurring on that day. Friday was also popular, accounting for 29 percent of listings at CBL and XBL.

Beyond knowing seasonal and daily trends, you can take a number of steps to ensure that your practices don't emulate a spammer's and land you on a blacklist this summer:

  • Clearly set expectations about your email program during the sign-up process and use the right permission level. It's important to be able to track and demonstrate subscriber consent.
  • Maintain consistent volume over the same set of dedicated IPs.
  • Create business rules to manage frequency across subscriber segments. When determining how often to send, take into account these subscribers' overall engagement with email, age on the list, email acquisition source, recent activity with email, and recent website and purchase behaviors.
  • Avoid using domains and hyperlinks that are commonly associated with spam as part of URL blacklists (like SURBL). For a list, visit
  • Use a tool that alerts you when your IPs have been blacklisted (and tells you why) so you can start fixing the problem immediately. Return Path's Reputation Monitor does this, but there are others, too.
  • Maintain good list hygiene, removing unknown users and those making spam complaints and unsubscribe requests, using a CAPTCHA form for sign-ups, implementing proper bounce handling processes, and removing absolutely inactive subscriber segments that could include spam traps.

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