A REST API fails synchronously — you get a 4xx and handle it on the spot. Email fails on a delay: the send call returns success, and the actual failure shows up minutes later as a bounce. An autonomous agent that only handles the first kind of error is half-built, and the half it's missing is the half that gets its sending paused. Here's the full failure surface for an email agent, and the retry design that survives it.
The context: agents built on Agent Accounts — hosted mailboxes the agent owns, currently in beta — where the agent itself decides when to send. That autonomy is exactly why error handling can't be an afterthought: a human notices when replies stop landing; a loop doesn't.
Channel one: errors at send time
Some failures do come back synchronously on the send request, and each maps to a different response from your code:
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