Once you scale beyond a few hundred thousand sends, “do everything in HubSpot” starts to creak. Licensing models, send-volume spikes, multi-brand setups, and stricter deliverability controls demand more muscle. The right move isn’t to abandon HubSpot—it’s to use HubSpot as the brain for segmentation, journeys, compliance and reporting, while an external ESP/MTA provides the raw horsepower to deliver at scale. Done right, you reduce cost per send, improve inbox placement, and keep one clean source of truth for Marketing, Sales, and CS.
Typical drivers include cost and throughput (dedicated IPs, parallel sending), deliverability governance (separate pools for marketing vs. transactional), and specialized enterprise needs such as sub-accounts, granular webhooks, or strict data-residency rules. The key is architecture: HubSpot should still decide who gets what and when; the external tool simply ships the mail.