When mail is rejected by the receiving server, or bounced, it may indicate a temporary issue or something more permanent. Continuing to mail recipients who consistently bounce can cause issues with deliverability to recipient inboxes, a drop in sender reputation, and even blocklistings. To avoid these potential negative outcomes, we recommend customers implement automatic suppression rules to ensure recipients that repeatedly bounce are removed from the regular sending rotation.
The most common reason to auto-suppress bounced recipients is a response indicating the address doesn't exist. However, other persistent problems that are unlikely to be resolved could also lead to long-term mail rejections and a need for suppression. Each situation is different, and not all senders will follow the same guidelines when determining how to handle these suppressions.
For this guide, we've put together a list of the most common temporary bounce categories and how we recommend handling suppressions for them. (If you'd like more general insight into these and other bounce categories before diving into our recommendations, check out Understanding Bounces.)
Bounces of this type should always be suppressed, generally after a single occurrence. Invalid Recipient bounces indicate the recipient address does not exist on that mail server.
DNS Failure bounces should also be suppressed, but not necessarily after a single bounce. These bounces indicate our mail servers were unable to find the required DNS records to indicate how to deliver mail to the domain in question. Occasionally these bounces can be caused by localized DNS issues or a widespread problem with a DNS resolver or other internet resource, although both are relatively rare. We generally advise suppressing these after no more than 3 consecutive occurrences.
Spam Block / Spam Content
These bounces leave a bit more room for interpretation and require additional caution with auto-suppression rules. The recipient address is most likely still good, but the mail is being blocked based on the message content or sender reputation. In many cases, these bounces can be resolved by reaching out to a mailbox provider or modifying body content to remove troublesome content like certain third-party links. We typically recommend only suppressing these if the spam block is persistent after reasonable attempts to resolve the underlying cause. If you'd like to automate this suppression, we recommend logic that requires multiple occurrences in a set number of days without a delivery -- 5 consecutive bounces in 30 days is a good starting point. This allows time to attempt to address the cause for the bounces before the automated suppression takes effect.
Generic Bounce & Undetermined
Bounces that are classified as Generic or Undetermined can also create challenges when building suppression rules. These are typically bounces for which the receiving server returns an SMTP response that is unclear (or returns no response text at all). In some cases these are spam-related bounces, but the bounce response returned by the receiving server does not give us enough detail to properly categorize them.
If you're new to MessageGears or have recently made changes to your email program -- like adding a new source of recipients or shifting your branding/content, we recommend avoiding auto-suppression for these two bounce categories until you've addressed any delivery or reputation issues driven by those changes. Even after you're sure you've resolved any contributing issues, we advise you to exercise caution and set a slightly higher tolerance for these types of bounces. A commonly-used threshold is 5 consecutive bounces without a successful delivery, although this may need to be raised for more frequent senders.
It's generally a best practice to suppress these after multiple occurrences, but the specific settings will depend on your mailing pattern. Is it possible (or even likely) someone would receive 3 messages in a single day (including transactional/triggered)? If so, we strongly advise bumping this to something considerably higher (potentially 5, 7, or 10) since we know they are fairly likely to engage with an email moving forward. If recipients get just a message or two a week, you can start with a lower total like 2-4 occurrences. One frequently used suppression method is specifying the number of days within which these bounces occurred along with the number of bounces, e.g. 7 consecutive bounces within 14 days.
Zero Successful Deliveries
Regardless of the bounce type, there is one additional suppression we recommend for all senders: Zero Successful Deliveries. In some cases, addresses that are not owned by a real recipient may end up in your list. These include abandoned addresses, typo domains (e.g. gnail.com or yhaoo.com), sensor networks, and spam traps. While these addresses will typically bounce, the bounce category or SMTP response may vary, and the addresses may even record engagement in the form of open pixel activation. To ensure your suppression captures these addresses, you should employ logic that identifies any address that has zero successful deliveries after multiple renders/attempted deliveries over an extended period of time.
You'll want to craft the rule to avoid new-to-list recipients, and we suggest limiting to recipients who had at least 3-5 attempted deliveries during a 6-month span. It's also a good idea to start with a higher threshold for attempted deliveries and gradually decrease over time. We often recommend starting with a rule along the lines of "10 attempted deliveries in the past 6 months with 0 successful deliveries recorded."