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.