trim all strings in an array

You could also replace all occurrences of spaces, and so avoid the foreach loop:

string email = "[email protected], [email protected], [email protected]";    
string[] emails = email.Replace(" ", "").Split(',');

Either one of the following would work. I'd recommend the first since it more accurately expresses the joining string.

string[] emails = email.Split(new string[] { ", " }, StringSplitOptions.None);
string[] emails = email.Split(new char[] { ' ', ',' }, StringSplitOptions.RemoveEmptyEntries);

emails.Split(',').Select(email => email.Trim()).ToArray()