Inbox Monitor performs blocklist checks for issues that are known to influence deliverability at Gmail and Yahoo! as part of the Inbox Optimizers. Because these checks are performed on every test, Return Path focuses on the most important ones.

The blocklists used in Inbox Monitor include:

• Spamhaus Policy Block List (PBL) for sending IP addresses that are blocklisted
• Spamhaus Domain Block List (DBL) for From domains and links in email content that are blocklisted