Determines envelope MAIL FROM address to be send to destination server.

A String value.  

object.MailFrom [= value]

The MailFrom Property syntax has these parts:

MailFrom property holds email address of the sender, as it will be provided to 'MAIL FROM' command when message is sent to destination server. This doesn't have to be same as 'From' field used in email. Technically, once relay connects to the server, communication goes like this:

220 weonlydo.com ESMTP; Fri, 15 Aug 2003 02:02:45 +0200
HELO weonlydo.com
250 weonlydo.com Hello weonlydo.com, pleased to meet you
MAIL FROM: kreso@weonlydo.com
250 <kreso@weonlydo.com>, Sender ok
RCPT TO: general@weonlydo.com
250 <general@weonlydo.com>, Recipient ok
DATA
354 Please start mail input.
this is a test
.
250 Mail queued for delivery.
quit
221 Closing connection. Good bye.

As you can see, line marked with red color is used from MailFrom property.

You may want to put MailFrom value between < and > characters, because some servers will reject the message if email address does not contain them. So, if you set

Relay.MailFrom = "<joe@usa.net>"

server will accept it ok.