Regex for detecting emails in text
string sRegex = @"([w-]+(.[w-]+)*@([a-z0-9-]+(.[a-z0-9-]+)*?.[a-z]{2,6}|(d{1,3}.){3}d{1,3})(:d{4})?)";

Regex Regx = new Regex(sRegex, RegexOptions.IgnoreCase);

string sContent = "ttt <a href='mailto:someone@example.com'>someemail@mail.com</a> abc email@email.com";

"ttt <a href='mailto:someone@example.com'>someemail@mail.com</a> abc <a href='mailto:email@email.com'>email@email.com</a>";

someemail@mail.com
email@email.com


public void Test()
{

    Regex pattern = new Regex(@"b(?<!mailto:)([w-]+(.[w-])*@([a-z0-9-]+(.[a-z0-9-]+)?.[a-z]{2,6}|(d{1,3}.){3}d{1,3})(:d{4})?)");
    MatchCollection matchCollection = pattern.Matches("ttt <a href='mailto:someone@example.com'>someemail@mail.com</a> abc email@email.com");
    foreach (Match match in matchCollection)
    {
        Debug.WriteLine(match);
    }
}

Regex pattern = new Regex(@"(?<!mailto:)b[w-]+@[a-z0-9-]+(.[a-z0-9-])*.[a-z]{2,8}b(?!</a)");
MatchCollection matchCollection = pattern.Matches("ttt <a href='mailto:so1meone@example.com'>someemail@mail.com</a> abc email@email.com");
foreach (Match match in matchCollection)
{
    Debug.WriteLine(match);
}