Optional ReadonlydeliveryRequests an RFC 2852 DELIVERBY deadline. The server must advertise
DELIVERBY, and for mode R the interval must meet its advertised
minimum.
Optional ReadonlydsnRequests the RFC 3461 RET DSN return type. The server must advertise DSN.
Optional ReadonlyenvidSets the RFC 3461 ENVID envelope identifier. The server must advertise DSN.
Optional ReadonlyextensionAdditional MAIL FROM parameters. Names are upper-cased and used verbatim;
a parameter without a value is emitted as a bare keyword. A BY entry is
ignored when SendOptions.deliveryBy is set.
Optional ReadonlyrequireRequests the RFC 8689 REQUIRETLS parameter, requiring TLS on every
delivery hop. The submission server must advertise REQUIRETLS or the
send fails with an SMTPError.
Optional ReadonlysignalCancels the submission.
Per-call options for Client.send and Client.sendRaw.