When should a team build?
A team should build when the workflow depends on product-specific behavior, custom scoring, or cross-system handoffs that off-the-shelf tools do not handle cleanly.
The build does not need to be huge. Often the right move is a small layer inside tools the team already owns.
When should a team buy?
A team should buy when the need is common, stable, and well served by a vendor. Deliverability, core ESP sending, CRM records, and analytics views usually belong in established products.
The custom layer should connect those products around your lifecycle motion.
What is the practical test?
Ask whether the missing capability is about your unique customer behavior. If yes, a focused build may create more value than another subscription.
Ask whether the team can own it after launch. If no, either shrink the scope or choose a vendor.