Assigning a value to a class variable
If you are sure that configuration.EmailFromAddress will always be valid or null, I'd suggest using the null-coalescing operator (??)

SystemUser user = (SystemUser)Session["CurrentUser"];
Data.Configuration configuration = db.Configurations.Single(c =&gt; c.ID == user.ID);
var fromAddress = configuration.EmailFromAddress ?? "noreply@test.com";
MailAddress from = new MailAddress(fromAddress, "Test");


In the case that configuration.EmailFromAddress can also be empty, I'd use the ternary operator

SystemUser user = (SystemUser)Session["CurrentUser"];
Data.Configuration configuration = db.Configurations.Single(c =&gt; c.ID == user.ID);
var fromAddress = String.IsNullOrEmpty(configuration.EmailFromAddress) ? configuration.EmailFromAddress : "noreply@test.com";
MailAddress from = new MailAddress(fromAddress, "Test");