I'm using Services.Mail.Mail.SendMail(strFrom, strTo, CC, BCC, Services.Mail.MailPriority.Normal, MsgSubject, Services.Mail.Mailformat.Html, System.Text.Encoding.UTF8, MsgBody, Attachment, SMTPServer, SMTPAuthentication, SMTPUsername, SMTPPassword) in a custom module installed on DNN 4.x

I'm finding that it only succeeds to send email, if strFrom is a valid user account on the SMTP Server.  I'm trying to allow users of my website to send a message with a link to our site to a friend.  Thus I want strFrom to be their email address which obviously won't be on our server.  When I try this, SendMail returns Mailbox unavailable. The server response was: No such user here. 

Is this a bug in SendMail, or am I missing something?

Thanks for any help,

Dave