Should I use a subdomain for cold email instead of my root domain?

Generally yes, for exactly this reason — cold outreach carries more deliverability risk than regular 1:1 business email (higher volume, more likely to get spam complaints), and if it's on the same domain as your regular email, a reputation hit from cold outreach can drag down deliverability for your everyday business email too. Using a dedicated subdomain (e.g. hello.yourcompany.com) isolates that risk — if the subdomain's reputation takes a hit, your main domain and regular email stay unaffected.