SMTP reference

Connection

Item	Value
Ports	587 STARTTLS, 465 implicit TLS
TLS	Required, version 1.2 or newer
Authentication	PLAIN or LOGIN, after TLS only
Connections	20 per port; 30 seconds idle timeout; 60 seconds to finish DATA
Failed logins	Throttled after 10 failures per address per minute

Message limits

• One recipient per message. A second RCPT TO is refused with 552 5.5.3.
• From must exactly match the approved sender.
• Body: text/plain, text/html, or one multipart/alternative with one of each. UTF-8 or US-ASCII.
• No attachments, nested MIME parts, SMTPUTF8, DSN or REQUIRETLS.
• Up to 2 MiB per message and 1 MiB of decoded body. Lines up to 998 bytes; headers up to 32 KiB and 100 fields.
• Headers that claim routing or authentication (Received, Return-Path, DKIM-Signature, Authentication-Results and similar) are refused. Bcc and unknown custom headers are dropped.
• X-Email-Idempotency-Key: optional, 16–128 printable characters.

Reply codes

Code	Meaning	Do
250	Stored and queued	Nothing
451 4.3.0	Temporarily unavailable or uncertain	Retry the identical message with the same key
452 4.2.2	Allowance used up	Wait or raise your plan
454 4.7.0	Too many failed logins	Wait, then check the credential
550	Idempotency key reused with different content	Use the original content, or a new key for a new message
other 5xx	Bad credential, sender or content	Fix and resend

To confirm before launch

These limits are from the current implementation and may change for the hosted service.