stackoverflow.com/questions/5465090/how-to-do-a-web-config-transform-for-smtp
From:
<mailSettings>
<smtp from="no-reply@test.com" deliveryMethod="SpecifiedPickupDirectory">
<specifiedPickupDirectory pickupDirectoryLocation="C:\temp"/>
<network host="localhost"/>
</smtp>
</mailSettings>
To:
<system.net>
<mailSettings>
<smtp deliveryMethod="Network">
<network host="smtp.mysite.com" userName="myuser" password="mypassword" />
</smtp>
</mailSettings>
</system.net>
Via:
<system.net>
<mailSettings>
<smtp deliveryMethod="Network" xdt:Transform="Replace">
<network host="smtp.mysite.com" userName="myuser" password="mypassword" />
</smtp>
</mailSettings>
</system.net>