I swapped to using the example outlined in my initial post on this question http://community.wolfram.com/groups/-/m/t/494598?p_p_auth=hlekduQ2
I found that changing my code to the below also provided the required effect.
SendMail[
"To" -> "l.j@domain.com",
"Subject" -> "Example Message",
"Body" -> "My text",
"From" -> "l.j@domain.com",
"Server" -> "mailrelay.sub.domain.com",
"UserName" -> "",
"Password" -> "",
"PortNumber" -> 25]
