Draft API for `smtp_request` endpoints for ICP canisters

smtp_request.did

type Address = record {
  // Complete address.
  //
  // Example: "alice@example.com"
  serialized : text;

  // Index into `serialized` before the '@'.
  //
  // 8-bit index works here since RFC 5321 limits the length of the local part to 64 characters.
  at_start : nat8;
};

// Examples:
//  record { name="From"; value="alice@example.com" };
//  record { name="To"; value="verify@id.ai" };
//  record { name="Subject"; value="Your verification code" };
//  record { name="DKIM-Signature"; value="v=1; a=rsa-sha256; d=example.com; ..." };
type Header = record {
  name : text;
  
  // The decoded value of the header, as specified by the sender.
  //
  // Non-ascii header values must be decoded via RFC 2047 by the caller.
  //
  // Example: "José Pérez"
  // 
  value : text;
};

// Simple email envelope representation
type Envelope = record {
  // The envelope sender address.
  //
  // More reliable than the "From" header, which can be easily spoofed.
  bounce_address : Address;

  // The adddresses of the envelope recipients
  to : vec Address;
};

type Message = record {
  // Raw RFC 5322 headers
  headers : vec Header;

  // We do not support MIME, only simple text/plain messages, so the body is just a blob of bytes.
  body : blob;

  // This is typically not available to the email recipient, but is used by the SMTP server
  // to determine where to send emails.
  envelope : Envelope,
};

type SmtpRequest = record {
  message : opt Message;
};

type SmtpRequestError = record {
  code : nat64;
  message : text;
};

// Used for logging on the SMPT adaptor side.
type SmtpResponse = variant {
  ok : record {};

  // Errors
  err : SmtpRequestError;
};

service : {
  smtp_request : (SmtpRequest) -> (SmtpResponse);
}