Email Deliverability Best Practices: Suppression Lists and Blocklists

What is suppression?

A suppression list is a list of email addresses which have been suppressed from inclusion in commercial emails. Unsubscribing from a mailing list is a type of suppression opt-out mechanism that everyone is familiar with, but there are other modes of suppression as well.

Unsubscribe Suppression

Unsubscribe suppression takes place when a recipient directly opts-out of the mailing. This occurs generally through an unsubscribe link included in the email. It's helpful for senders to maintain a list of opt-outs in their own database or indicate for those contacts when they have opted-out of specific mailings. Sending to an email address which has already opted-out of your mailings is a bad practice and often leads to spam complaints, which can have a negative effect on your sending reputation and deliverability. Prompt suppression of unsubscribed email addresses is very important.

Hard Bounce Suppression

Hard bounce suppression occurs when a recipient email address hard bounces for a permanent reason, such as an invalid email address. When hard bounces occur, that means that email address is undeliverable, and we suppress these to discontinue sending to an address that is known to be undeliverable. Suppressing these email addresses helps to avoid inflated bounce rates or low delivery rates in future mailings by cleaning up the recipient list and eliminating undeliverable addresses.

What about soft bounces?

MessageGears will retry soft bounces (i.e. "mailbox full") for 24 hours. If the mail cannot be delivered, the email address will not be added to hard bounce suppression but will appear counted as a bounce within the specific job's activity stats.

Complaint Suppression

As with unsubscribes, spam complaints should result in the email address being immediately suppressed. The primary difference between unsubscribes and spam complaints is that spam complaints carry extra weight and they express to the recipient's email service provider that the sender did not gain their permission to email them. High complaint rates are likely to cause you deliverability issues moving forward, and can be avoided by sending to a well-maintained recipient database based primarily on opt-ins. When a recipient knows who you are and why you're sending to them, they are unlikely to report your emails as spam.

Complaints are automatically handled by the MessageGears system through Feedback Loops with email and internet service providers. A complaint will immediately suppress that address for the Account the mailing was sent from. By maintaining a strict complaint suppression list, MessageGears helps to ensure that recipients aren't getting unwanted mail, while senders are made aware of issues with their sends in order to help with future deliverability.

MessageGears Suppression vs Custom Suppression

With MessageGears, the system by default suppresses unsubscribes, hard bounces, and spam complaints at the Account level. Since we are an Enterprise solution, customers sending both marketing and transactional email in the same account can run into issues with the Account-level suppression lists for unsubscribes and hard bounces. Spam complaint suppression is always managed by MessageGears, but mirroring those opt-outs in your own database is important for the cleanliness of your data. When it comes to unsubscribes in particular, Account-level suppression can cause issues. Say a customer of your organization may want to unsubscribe from a marketing email, for example, but transactional emails like receipts or password resets are still important items for them to receive separately. In cases like this, you may choose to manage your mailing lists within your own database, including suppression. In such cases, you can work with MessageGears Support to customize your suppression settings.

What is a blocklist?

A blocklist is a list created and maintained to monitor email sending behaviors, to help email service providers (ESPs) and filtering products to more effectively block spam and fraudulent email from their customers' inboxes.

It's important to keep in mind that just because you see your domain or sending IP on a blocklist doesn't mean that's having a large effect on your email deliverability. It's certainly not a good thing, but there are a large number of blocklists and each recipient domain's email server system usually only checks one or two as a part of their email filtering protocol. Blocklists don't necessarily block your email - the receiving server's spam filter implementation would be where the block originates.

There are two different types of blocklists. IP-based blocklists contain lists of IP addresses deemed spammy, which are updated and checked in real-time. This type of blocklist is the most commonly used. The other type is domain-based, and lists of this type contain domain names contained in emails identified as spammy.

Some acronyms related to blocklists

What blocklists are reputable? What blocklists aren't reputable?

To address this, we want to first define what we mean by "reputable" here: reputable in the email blocklist world primarily means that the blocklist is actively maintained. Essentially anyone can create a blocklist, and as a result they don't all stay maintained over the years. You may do an internet search on a sending IP and find that it's on a blocklist, but it's important to check that listing's website. An IP could be listed on a blocklist which has been dormant and unused for years. In these cases, it's unlikely that blocklist is being used for filtering by any existing mail servers.

These are some commonly used and reputable blocklists:

• Spamhaus (SBL, CBL, XBL, DBL)
• Barracuda
• SpamCop (SCBL)
• Invaluement (ivmURI, ivmSIP, ivmSIP/24)
• URIBL
• SURBL

If you find references to other blocklists in your email stats or have any concerns about a blocklisting issue, please reach out to Support!

Spam Traps and other issues with a poor-quality list

Old, stale, or purchased lists often contain dormant email addresses turned into spam traps by blocklists and public ESPs. Hitting one or many spam traps on the same receiving domain is the quickest way to find your sending IPs or domain on a blocklist.

Maintaining a clean, updated, and permission-based list which you send to at least every 3-6 months is the best way to avoid having spam traps on your recipient list. Stale emails will run a higher risk of becoming spam traps, as well as receiving complaints.

Spam Complaints

If the percentage of recipients exceeds their filtering system's blocklist's threshold, those complaints would land the sender IP or domain on that blocklist. We don't know exactly what these thresholds are, but the best way to avoid being reported as spam is to maintain engagement with your recipient list so they don't believe your mail is unsolicited. Additionally, making sure to send only to permissions-based recipients list is very important for deliverability, reputation, and compliance with international spam laws such as CASL and CAN-SPAM.

How do I get delisted or removed from a blocklist?

Each blocklist has a different delisting process - some of them are automatic, some of them allow delisting by request, and others keep their processes a bit of a mystery to ESPs in order to maintain the integrity of their process.

When it comes to automatic processes, we can't directly address those to request delisting.

SpamCop, for example, has a 24 hour automatic delisting process. Often by the time we hear about an issue, that IP has already been delisted. Other blocklists with automatic delisting are a bit more close-lipped about their timeframes, and the only way to make sure delisting is possible is to identify and correct any poor sending behaviors which likely caused the listing.

Certain blocklists like Barracuda do allow us to request delisting. In these cases we need to provide a description of the cause of the listing and explain how we are seeking to rectify the issue or behavior.