Should I use subdomains or separate root domains?

The choice between subdomains and separate root domains involves tradeoffs in isolation versus management.

Subdomains (outreach.company.com):

Easier DNS management (same registrar, same zone)

Maintains brand connection

Can have separate SPF, DKIM, DMARC

Some reputation isolation, but parent domain can be affected

Severe problems (Spamhaus DBL) can impact root domain

Separate root domains (companyhq.com, trycompany.io):

Stronger reputation isolation

Independent registration and management

Completely separate reputation profiles

More setup and maintenance overhead

Can still be connected by mailbox providers if patterns are obvious

Recommendations:

For cold outreach with significant risk, use separate root domains

For lower-risk segmentation, subdomains may suffice

Always keep transactional email on separate infrastructure from cold outreach

Practical approach:

Use subdomains for segmentation within generally safe sending

Use separate root domains for cold outreach that could damage main brand

The greater the risk, the stronger the isolation should be.