When a request returns
429, honor Retry-After and use exponential backoff with jitter. Do not open an unbounded retry or polling loop. RateLimit-Reset is a UTC Unix timestamp for the authoritative allowance window; it is not permission to ignore short-window protection.
Current rolling five-minute inbound defaults are 20 messages or 50 MiB per email and 100 messages or 250 MiB per account. SMTP can temporarily refuse traffic above these safety ceilings. Such refusal does not consume public API allowance.
Current content ceilings are 10 MiB raw MIME, 128 KiB MIME headers, 30 nesting levels, 256 multipart delimiter lines, 512 KiB each for parsed text and sanitized HTML, 10 attachments, 5 MiB per attachment, and 8 MiB total attachment data.
These operational safety defaults are not pricing promises and can be tuned after isolated evidence.