List < string > strList = new List < string > ();
strList.Add("@gmail.com");
strList.Add("@crossware.co.nz");

if(EmailProperties.ContainsKey("SMTPRCPTTO"))
{
foreach(string strItem in strList)
{
if(EmailProperties["SMTPRCPTTO"].IndexOf(strItem, StringComparison.OrdinalIgnoreCase) >= 0)
{
return true;
}
}
}
return false;