Ever wondered why you got an email with "to:" being some other address and not yours? Perhaps you got an email stating "to: Undisclosed recipients:;", why? you might have asked.
To simplify the explanation here the RCPT command using the verb “to:” is used to direct the email to a specific address. In the last mile the email will be received by the addressee but only the "to:" header if present will be accessible. If it is missing you get "to: Undisclosed recipients:;" and if it is set you get whatever it says. Clearly you can use a different email address there which will generate in some cases a heck of confusion ;-). You can confirm this yourself just by using telnet as usual for SMTP: