/SUBADDRESS=subaddress

    Specifies a subaddress to be set. Subaddresses are addresses
    that are explicitly qualified by the username they are associated
    with; they have a local-part of the form username+subaddress.
    The /SUBADDRESS and /USER qualifiers are mutually exclusive.
    Note that subaddresses can be set by using a username of the form
    username+subaddress in conjunction with /USER; this qualifier is
    simply a convenience for users who want to set subaddresses for
    themselves.