Suppression List Checker

The list you must not send to

Every outreach programme accumulates a second list beside the prospect list: people who unsubscribed, addresses that hard bounced, accounts already in the pipeline, existing customers, and companies someone senior asked you to leave alone. Sending to any of them is avoidable, and the check takes seconds. What makes it fail in practice is that the two lists rarely spell the same person the same way.

Exact matching misses most of the overlap

The same person is Sam.Reed@Acme.com in your CRM export and sam.reed@acme.com in the unsubscribe file. A plus tag added at signup makes a third variant. Comparing raw strings clears none of those, which is how someone who opted out ends up in the next campaign despite a suppression step existing.

Dots only matter at some providers

Gmail ignores dots in the local part, so first.last and firstlast are one mailbox. Almost nowhere else does, so stripping dots universally would merge two genuinely different people. The alias handling here follows the provider rather than applying one rule everywhere.

Sometimes the unit is the company

When a company asks to be left alone, or an account is owned by someone else on the team, suppressing one address is not enough. Matching on the domain removes every contact there, which is the correct behaviour for do-not-contact requests and account conflicts, and the wrong one for ordinary unsubscribes.