Use regex to parse the text.
using System.Text.RegularExpressions;
...
string input="someone@somedomain.com.au is an email address... and someone@anotherdomain.com is yet another one.";
string pattern="\\b[\\w\\.-]+@[\\w\\.-]+\\.\\w{2,4}\\b";
var matches =Regex.Matches( input,pattern);
string[] addresses = matches.OfType<match>().Select(m => m.Value).ToArray();
your input string would be the contents of your text file or in the case of excel you could read each cell into a StringBuilder variable.
ATG