Separate email ids after .com with comma (,) using C# in ASP.Net

In my application i am getting email ids as shown below without getting seperated with comma i need to seperate the with comma after .com using c# code.How can i do this

i am getting the email id's as shown below nitya@gmail.combhavya@gmail.com

and i need to seperate them after .com using c# code.How can i do this

but i am getting two commas when comibining below is my c# code  

string Mailc1 = Names[Names.Length - 1].ToString();
string Mailb = txt1.Text;
Mailc = Mailbcc.Replace(".com", ".com,") + Names[Names.Length - 1].ToString();

nitya@ho.elicohcs.com,,bhavya@ho.elicohcs.com,nalini@elicohcs.com