List<string> list=new();
list.Add("xx1");
list.Add("xx2");

//第一种
var mailStr = "";
foreach (var mail in list)
{
    mailStr += "'" + mail.ToString() + "'" + ",";
}
mailStr = mailStr[0..^1];
Console.WriteLine(mailStr);

//第二种
Console.WriteLine(string.Join(",", list.Select(o => $"'{o}'")));