The To property returns the list of addresses that received
a copy of the current message. If there is no current message, or the
To header field is not defined, then this property will return an
empty string.
Setting the To property creates or changes the value of the
To header field in the message and specifies additional recipients of
the message. Multiple addresses may be specified by separating them
with a comma. Each address must conform to the standard Internet
address format.