How to show error message if user misspelled gmail.com(like gaiml.com)

This is a very simple regular expression, but it will show the principle.
(There are plenty of other variants out there)

The problem with adding hard coded names into the expression is that you need to update the expression in order support a new one.

A maybe better way is to have a list with valid names. This list can be configurable and easy to change later on.

Regex MailExpression = new Regex(@"(?<name>\S+)@(?<provider>\S+)\.(?<code>\S+)");
// Hard coded here, but should be read from config file
List<string> validProviderNames = new List<string> {"yahoo", "gmail", "whatever" };

string testAddress = "harry.hacker@yahaa.com";
Match m = MailExpression.Match(testAddress);
if (m.Success)
{
    string providerName = m.Groups["provider"].Value;
    if (!validProviderNames.Contains(providerName.ToLower()))
    {
        throw new Exception(String.Format("The provider name '{0}' is not valid.", providerName));
    }
    else
    {
        // Do som stuff
    }
}

The solutions above are crap: there are far to many valid domains that you could check them with a regular expression or a list of "valid" domains.
So look what others do: have the user repeat his email address, then send him a confirmation email with a custom link for activating his account.
If the two email addresses differ, you can show him an error message. If the user "insists" on typing the same typo twice, well, he won't receive your confirmation email, and then you can savely delete his account within a few hours.