Short answer
HubSpot's KB: clicks, opens and spam complaints feed your reputation. Monitor the last 30 days of deliverability, turn on double opt-in for forms, and warm up sending by ramping volume gradually.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Time zone sending and A/B tests need Marketing Hub Professional or Enterprise.
3. Related
Spam checklist and double opt-in.