Before any high-stakes send, run these three tests:
1. Seed list test. Inbox OSS includes a seed-list service that sends to 30+ test mailboxes across major providers, then reports actual inbox-vs-spam placement. Available on Essentials and above. Run it on the exact campaign you are about to send.
2. Content scoring. SpamAssassin score should be under 3. The dashboard's template editor shows the score live as you edit. Anything above 5 will land in spam at most providers.
3. Authentication check. Run mail-tester.com on your test send. 10/10 score means SPF, DKIM, DMARC are all aligned. Anything less, fix before sending.
For critical sends (product launches, transactional changes), run all three. The cost is minutes; the cost of a bad send is weeks of reputation recovery.