Gmail and Outlook do not make inbox decisions based on one single signal. They look at a mix of reputation, authentication, user behavior, and list quality.
What matters most
Sender reputation
If your domain or sending setup has a history of bounces, complaints, or erratic volume, inbox placement gets harder.
Authentication
SPF, DKIM, and DMARC are foundational. If those are missing or broken, trust drops quickly.
Engagement
If people open, click, reply, and keep your emails, that helps. If they ignore or complain, that hurts.
List quality
Sending to bad addresses creates bounce signals that providers notice immediately.
What you can control
You cannot control every provider decision, but you can control the basics:
- keep authentication healthy
- send consistently
- avoid sudden volume spikes
- make unsubscribe easy
- clean the list before important sends
MailCull fits the list-quality part of this workflow. It helps you remove the clearly bad addresses that cause preventable bounce damage.