Historically an Internet Service Provider (ISP) was an entity that provided a connection to the internet for their customers, such as Comcast or Verizon. Now we consider ISPs to be anyone who provides you with an email address, like Outlook, Gmail, Yahoo, Aol, and Comcast or Verizon. Some less widely used term for an ISP are "email provider" and "mailbox provider."
Getting your email into the inbox is a top priority, and it's important to understand how and why an ISP blocks emails, and what you can do about it if it's affecting you:
Why ISPs Block Emails
ISPs use spam-blocking measures to help sort through incoming email and identify the spam to keep their own customers happy. Large ISPs even boast about being able to keep their users' inboxes "spam free." Once they've identified a message as unwanted, or as spam, ISPs do their best to block all similar messages.
Sometimes these measures can inadvertently block the delivery of legitimate emails because the ISP has no way of knowing if the messages were legitimately requested in the first place. When a legitimate email is blocked as spam, it's called a "false positive."
How ISPs Block Emails
ISPs use reputation systems to filter mail, taking data from many external sources as well as their own internal data, to determine if your email should be considered spam. Reputation systems are more robust than traditional content filtering and email block lists, that often use a single bit of content or images. Reputation systems instead use many data points to score incoming mail.
Of course all ISPs are different, but the following is a list of data points they have available for use in their reputation systems:
- Email Content - This includes the subject line, preheader, and body content. The content is scanned for keywords and phrases that trigger spam filtering. You have control over what you put into your email, so make sure to run a spam check before you hit send.
- Images - Images can be scanned for similarity to images that have previously been reported. Emails are also scanned for all-image or image-only content. Make sure your email has some text content to prevent it from triggering a spam filter.
- URL Block Lists - If an URL in your email is listed on a blacklist, it can get your email marked as spam. There are several public block lists that are designed to prevent access to malicious content; ISPs can also compile their own block list. You can prevent blocks by only linking to URLs for reputable web pages.
- Source IP for Your Email - When you send mail through Constant Contact, it’s sent through our mail servers and therefore is associated with our IP address. We work hard to protect our reputation as an email sender and actively monitor accounts to prevent spam activity. We’ve also built relationships with many ISPs so that we can act quickly to resolve issues associated with our IP address if they arise.
- Individual Recipient Preferences - This is based on what your contact chooses in their inbox settings. It can be anything from having your "From Email" address added to their safe-sender list, or having your email filtered to a specific folder or tab.
- Individual Recipient Actions - Some ISPs manage email based on the behavior of their customers. For example, if a user never opens your emails, the next one you send could be filtered automatically into the spam folder in favor of email from senders that get opened and read more often. If you notice a contact is frequently listed as “did not open” in your reporting, it may be time to rethink how frequently you send them email, figure out what content truly engages them, or remove them from your list altogether.
- Individual Past Spam Treatment - If an individual has marked your email as spam in the past, it's likely to be treated as spam going forward, even if the individual has signed up for your email list.
- Content Association - There could be nothing wrong with your email, but your message could look similar to a completely unrelated message that the recipient has marked as spam in the past.
What to Do When an ISP Blocks Emails
If you do discover that your Constant Contact email is being blocked, a good first step is to look at the prior engagement of the blocked address or addresses. If the contact was:
- Previously Opening Your Emails - It's likely something may have changed with the content of the email or the sending address, and you should contact our Support Team to help you troubleshoot the blocking. Having specific examples of contacts who were previously engaged prior to bouncing is useful prior to calling Support.
- Never Opening Your Emails - It's likely a problem with list hygiene. Removing disengaged contacts and keeping a clean list of engaged contacts helps to reduce blocks and improve your open rate.
We work with many ISPs and corporate mail administrators to successfully resolve issues with ISPs blocking emails. Once we have the opportunity to explain our business, they're willing to work with us to resolve the issue. We also speak with any mail service that isn't currently accepting email from Constant Contact. For smaller ISPs and domains, we've found that when the ISP hears from their own users who want to receive our email, it usually yields faster results, so we've created some form letters to help you communicate with the ISP directly.
