A narrow permission with an outsized effect
Send-as delegation grants a principal the ability to send a message that appears, to any recipient, to have come from a different identity — a shared team address, a colleague's mailbox, a system's own name — without granting that principal any of the identity's other access. It's meant to be narrow: the ability to speak as someone, nothing broader.
What makes it worth treating carefully is asymmetric: the recipient extends the same trust to the message that they would to one the original identity actually wrote themselves. A permission that's narrow in what it technically grants can still have an outsized effect, because its entire value lies in being indistinguishable from the real thing once it's used.
The historical lesson from open relays
Email infrastructure has a specific, costly history here: an open relay forwards mail for anyone without verifying the sender is actually authorized to route through it, and this was, for years, one of the most consequential and common authorization failures in the space. The general lesson outlasts the specific technology — any system that forwards or acts on a caller's behalf needs to verify the caller is actually entitled to use it that way, rather than treating a request simply reaching the system as evidence it's legitimate.