Basically, Disposable Addresses are similar to Plus Addresses that have been in SmarterMail for years, the only difference is they have a definable expiration, preventing any email from being received at that address after expiration. The purpose is entirely for one-time (or daily) use when signing up for services, or when needing a password recovery from an external service, etc. so that you do not receive marketing spam from their service. Neither Disposable Addresses or Plus Addresses are intended for sending email from as that could/would be a source of abuse as a delivery method for Spoofed Spam.