Return-Path
The envelope sender address used for bounces and SPF alignment.
Return-Path is the address that receives bounces and is checked for SPF. It can differ from the visible From address. For DMARC to pass via SPF, the Return-Path domain must align with the From domain.
reading this as early-stage startups
At an early stage the deciding factors are what the free tier actually covers, what the first paid invoice looks like, and how much of a founding engineer's week the setup costs. Feature depth that only matters at a million sends a month is noise until it is not.
Applied to return-path, that means weighing free tier, best published rate per 1,000, entry price, and developer experience ahead of the rest, against a waitlist, a launch announcement, onboarding email, and the first transactional sends.