For almost any capability a system needs, such as sending mail, storing files, running a queue, or authenticating people, there are three ways to get it. You build it, which means you own the design, the operation, the upgrades and the on-call phone for it forever. You buy or rent a running service, which means you own the integration and the bill and nothing else, and you give up control over its interface, its pace of change and its failure modes. Or you take an open component and run it yourself, which is a middle position where you own the operation but not the design. The choice is not about cost alone, and it is not about whether you could write it. Three questions decide it. Is this capability something your users would recognise as the reason they chose you, or is it plumbing every system needs? What does the exit look like if the supplier's price, terms or reliability change, and how much of your code would have to change with it? And what is the true cost of the built version, which is the initial work plus the operation for its whole life, not the first release. Building the thing that makes you different and renting everything else is the default the questions usually reach. You can now argue a build, buy, or rent decision on ownership of work and exit cost rather than on preference.
The build-vs-buy decision is a resource-allocation framework in software engineering for choosing whether to develop a capability in-house or acquire it from a third-party vendor, evaluated against a…