From Property

Gets or sets the e-mail address of the message sender.

public EmailAddress From {get; set;}

An EmailAddress object containing the e-mail address of the message sender.

The value of this property is taken from From header.

To specify it as a string, set msg.From.AsString value (assuming msg is MailMessage instance).

Exception Type	Condition
MailBeeInvalidArgumentException	value is a null reference (Nothing in Visual Basic).

The example is available in MailMessage topic.

MailMessage.From Property