Another option is to use Regex.Split. This is useful when the split sequences are more complex. For instance if spaces can also be part of the split delimiters such as:
"4,6,8 , 9\\n\\n4"
Then:
using System.Text.RegularExpressions;
var i = "4,6,8 , 9\n\n4";
var o = Regex.Split(i, @"[,\s\n]+");
// now o is:
// new string[] { "4", "6", "8", "9" }
Note that the regular expression used is "more accepting" - it ignored the empty "space" between the \n's and it would accept "4 6 8 9 4" just the same - so the above to to show a point: there is more than one way to skin a cat.
// input string
string input = "email@domain.com;email2@domain.com,email3@domain.com;";
// each character in this string will split it
string splitBy = ",;";
// do the split, remove any blank results
string[] result = input.Split(splitBy.ToCharArray(), StringSplitOptions.RemoveEmptyEntries);