For the unit-integration layer that tests transactional emails, we leverage Docker. Our incoming edge is when the upstream service is finished processing a message and hands it to us for delivery, and then our outgoing edge is actually communicating with someone's inbox. We don't actually want to set up a bunch of receiving MTAs and such, but we still need to test behavior at that layer. Our solution is still a work in progress, but it gets the lion's share of use cases covered so we can confidently refactor and push new features and know we did not break anything.
This Docker setup leverages DNSMasq for setting up MX and A records and ensures they point to running mock inbox sinks. These inboxes are configured from a base image with multiple options. We can specify that the sink's TLS certificate is expired or improperly set up, we can have them respond slowly or with given errors at different SMTP conversation parts. We can ensure that we are backing off and deferring email if the inbox provider says to do so. This detailed faking of the outside world allows us to automate all kinds of outside behavior and ensure that our services behave as expected.
We develop locally in Docker, as we just went into. Our docker-compose file spins up containers with fancy DNS settings and all our dependencies, allowing us to test the MTA against a variety of MX and TLS settings, alongside a variety of potential inbox responses and behaviors. Everyone uses their editor of choice and we often pair up on more complex tasks to prevent siloed system understanding.