@"^([w.-]+)@([w-]+)((.(w){2,3})+)$"

public bool IsValid(string emailaddress)
{
    try
    {
        MailAddress m = new MailAddress(emailaddress);

        return true;
    }
    catch (FormatException)
    {
        return false;
    }
}

string email = txtemail.Text;
Regex regex = new Regex(@"^([w.-]+)@([w-]+)((.(w){2,3})+)$");
Match match = regex.Match(email);
if (match.Success)
    Response.Write(email + " is correct");
else
    Response.Write(email + " is incorrect");

@"^[w!#$%&'*+-/=?^_`{|}~]+(.[w!#$%&'*+-/=?^_`{|}~]+)*"
+ "@"
+ @"((([-w]+.)+[a-zA-Z]{2,4})|(([0-9]{1,3}.){3}[0-9]{1,3}))$";

bool IsValidEmail(string strIn)
{
    // Return true if strIn is in valid e-mail format.
    return Regex.IsMatch(strIn, @"^([w-.]+)@(([[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.)|(([w-]+.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(]?)$");
}

Invalid: @majjf.com
Invalid: A@b@c@example.com
Invalid: Abc.example.com
Valid: j..s@proseware.com
Valid: j.@server1.proseware.com
Invalid: js*@proseware.com
Invalid: js@proseware..com
Valid: ma...ma@jjf.co
Valid: ma.@jjf.com
Invalid: ma@@jjf.com
Invalid: ma@jjf.
Invalid: ma@jjf..com
Invalid: ma@jjf.c
Invalid: ma_@jjf
Invalid: ma_@jjf.
Valid: ma_@jjf.com
Invalid: -------
Valid: 12@hostname.com
Valid: d.j@server1.proseware.com
Valid: david.jones@proseware.com
Valid: j.s@server1.proseware.com
Invalid: j@proseware.com9
Valid: j_9@[129.126.118.1]
Valid: jones@ms1.proseware.com
Invalid: js#internal@proseware.com
Invalid: js@proseware.com9
Invalid: js@proseware.com9
Valid: m.a@hostname.co
Valid: m_a1a@hostname.com
Valid: ma.h.saraf.onemore@hostname.com.edu
Valid: ma@hostname.com
Invalid: ma@hostname.comcom
Invalid: MA@hostname.coMCom
Valid: ma12@hostname.com
Valid: ma-a.aa@hostname.com.edu
Valid: ma-a@hostname.com
Valid: ma-a@hostname.com.edu
Valid: ma-a@1hostname.com
Valid: ma.a@1hostname.com
Valid: ma@1hostname.com

@"^([0-9a-zA-Z]([+-_.][0-9a-zA-Z]+)*)+"@(([0-9a-zA-Z][-w]*[0-9a-zA-Z]*.)+[a-zA-Z0-9]{2,17})$";

const String pattern =
   @"^([0-9a-zA-Z]" + //Start with a digit or alphabetical
   @"([+-_.][0-9a-zA-Z]+)*" + // No continuous or ending +-_. chars in email
   @")+" +
   @"@(([0-9a-zA-Z][-w]*[0-9a-zA-Z]*.)+[a-zA-Z0-9]{2,17})$";

var validEmails = new[] {
        "ma@hostname.com",
        "ma@hostname.comcom",
        "MA@hostname.coMCom",
        "m.a@hostname.co",
        "m_a1a@hostname.com",
        "ma-a@hostname.com",
        "ma-a@hostname.com.edu",
        "ma-a.aa@hostname.com.edu",
        "ma.h.saraf.onemore@hostname.com.edu",
        "ma12@hostname.com",
        "12@hostname.com",
};
var invalidEmails = new[] {
        "Abc.example.com",     // No `@`
        "A@b@c@example.com",   // multiple `@`
        "ma...ma@jjf.co",      // continuous multiple dots in name
        "ma@jjf.c",            // only 1 char in extension
        "ma@jjf..com",         // continuous multiple dots in domain
        "ma@@jjf.com",         // continuous multiple `@`
        "@majjf.com",          // nothing before `@`
        "ma.@jjf.com",         // nothing after `.`
        "ma_@jjf.com",         // nothing after `_`
        "ma_@jjf",             // no domain extension
        "ma_@jjf.",            // nothing after `_` and .
        "ma@jjf.",             // nothing after `.`
    };

foreach (var str in validEmails)
{
    Console.WriteLine("{0} - {1} ", str, Regex.IsMatch(str, pattern));
}
foreach (var str in invalidEmails)
{
    Console.WriteLine("{0} - {1} ", str, Regex.IsMatch(str, pattern));
}

Abc.@example.com
Abc..123@example.com
name@hotmail
toms.email.@gmail.com
test@-online.com

hello..world@example..com

using System.Text.RegularExpressions;

public static bool IsValidEmail(string email)
{
    return Regex.IsMatch(email, @"A[a-z0-9]+([-._][a-z0-9]+)*@([a-z0-9]+(-[a-z0-9]+)*.)+[a-z]{2,4}z")
        && Regex.IsMatch(email, @"^(?=.{1,64}@.{4,64}$)(?=.{6,100}$).*");
}

public bool IsValidEmailAddress(string s)
{
    if (string.IsNullOrEmpty(s))
        return false;
    else
    {
        var regex = new Regex(@"w+([-+.']w+)*@w+([-.]w+)*.w+([-.]w+)*");
        return regex.IsMatch(s) && !s.EndsWith(".");
    }
}

public static bool IsValidEmailAddress(this string s)
{
    var regex = new Regex(@"[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?");
    return regex.IsMatch(s);
}

myEmailString.IsValidEmailAddress();

var myPattern = Regex.EmailPattern;

/// <summary>
    /// Validates the string is an Email Address...
    /// </summary>
    /// <param name="emailAddress"></param>
    /// <returns>bool</returns>
    public static bool IsValidEmailAddress(this string emailAddress)
    {
        var valid = true;
        var isnotblank = false;

        var email = emailAddress.Trim();
        if (email.Length > 0)
        {
            // Email Address Cannot start with period.
            // Name portion must be at least one character
            // In the Name, valid characters are:  a-z 0-9 ! # _ % & ' " = ` { } ~ - + * ? ^ | / $
            // Cannot have period immediately before @ sign.
            // Cannot have two @ symbols
            // In the domain, valid characters are: a-z 0-9 - .
            // Domain cannot start with a period or dash
            // Domain name must be 2 characters.. not more than 256 characters
            // Domain cannot end with a period or dash.
            // Domain must contain a period
            isnotblank = true;
            valid = Regex.IsMatch(email, Regex.EmailPattern, RegexOptions.IgnoreCase) &&
                !email.StartsWith("-") &&
                !email.StartsWith(".") &&
                !email.EndsWith(".") &&
                !email.Contains("..") &&
                !email.Contains(".@") &&
                !email.Contains("@.");
        }

        return (valid && isnotblank);
    }

    /// <summary>
    /// Validates the string is an Email Address or a delimited string of email addresses...
    /// </summary>
    /// <param name="emailAddress"></param>
    /// <returns>bool</returns>
    public static bool IsValidEmailAddressDelimitedList(this string emailAddress, char delimiter = ';')
    {
        var valid = true;
        var isnotblank = false;

        string[] emails = emailAddress.Split(delimiter);

        foreach (string e in emails)
        {
            var email = e.Trim();
            if (email.Length > 0 && valid) // if valid == false, no reason to continue checking
            {
                isnotblank = true;
                if (!email.IsValidEmailAddress())
                {
                    valid = false;
                }
            }
        }
        return (valid && isnotblank);
    }

    public class Regex
    {
        /// <summary>
        /// Set of Unicode Characters currently supported in the application for email, etc.
        /// </summary>
        public static readonly string UnicodeCharacters = "À-ÿp{L}p{M}ÀàÂâÆæÇçÈèÉéÊêËëÎîÏïÔôŒœÙùÛûÜü«»€₣äÄöÖüÜß"; // German and French

        /// <summary>
        /// Set of Symbol Characters currently supported in the application for email, etc.
        /// Needed if a client side validator is being used.
        /// Not needed if validation is done server side.
        /// The difference is due to subtle differences in Regex engines.
        /// </summary>
        public static readonly string SymbolCharacters = @"!#%&'""=`{}~.-+*?^|/$";

        /// <summary>
        /// Regular Expression string pattern used to match an email address.
        /// The following characters will be supported anywhere in the email address:
        /// ÀàÂâÆæÇçÈèÉéÊêËëÎîÏïÔôŒœÙùÛûÜü«»€₣äÄöÖüÜß[a - z][A - Z][0 - 9] _
        /// The following symbols will be supported in the first part of the email address(before the @ symbol):
        /// !#%&'"=`{}~.-+*?^|/$
        /// Emails cannot start or end with periods,dashes or @.
        /// Emails cannot have two @ symbols.
        /// Emails must have an @ symbol followed later by a period.
        /// Emails cannot have a period before or after the @ symbol.
        /// </summary>
        public static readonly string EmailPattern = String.Format(
            @"^([w{0}{2}])+@{1}[w{0}]+([-.][w{0}]+)*.[w{0}]+([-.][w{0}]+)*$",                     //  @"^[{0}w]+([-+.'][{0}w]+)*@[{0}w]+([-.][{0}w]+)*.[{0}w]+([-.][{0}w]+)*$",
            UnicodeCharacters,
            "{1}",
            SymbolCharacters
        );
    }

w+([-+.']w+)*@w+([-.]w+)*.w+([-.]w+)*

public bool IsEmail(string text)
{
    Regex regex = new Regex(@"^(([w-]+.)+[w-]+|([a-zA-Z]{1}|[w-]{2,}))@" + @"((([0-1]?[0-9]{1,2}|25[0-5]|2[0-4][0-9]).([0-1]?[0-9]{1,2}|25[0-5]|2[0-4][0-9])." + @"([0-1]?[0-9]{1,2}|25[0-5]|2[0-4][0-9]).([0-1]?[0-9]{1,2}|25[0-5]|2[0-4][0-9])){1}|" + @"([a-zA-Z]+[w-]+.)+[a-zA-Z]{2,4})$");
    return regex.IsMatch(text);
}

public bool VailidateEntriesForAccount()
    {
       if (!(txtMailId.Text.Trim() == string.Empty))
        {
            if (!IsEmail(txtMailId.Text))
            {
                Logger.Debug("Entered invalid Email ID's");
                MessageBox.Show("Please enter valid Email Id's" );
                txtMailId.Focus();
                return false;
            }
        }
     }
   private bool IsEmail(string strEmail)
    {
        Regex validateEmail = new Regex("^[\W]*([\w+\-.%]+@[\w\-.]+\.[A-Za-z] {2,4}[\W]*,{1}[\W]*)*([\w+\-.%]+@[\w\-.]+\.[A-Za-z]{2,4})[\W]*$");
        return validateEmail.IsMatch(strEmail);
    }

if (!System.Text.RegularExpressions.Regex.IsMatch("<Email String Here>", @"^([w.-]+)@([w-]+)((.(w){2,3})+)$"))
        {
            MessageBox.show("Incorrect Email Id.");
        }

public class LoginViewModel
{
    [EmailAddress(ErrorMessage = "The email format is not valid")]
    public string Email{ get; set; }

public static bool ValidateEmail(string str)
{
     return Regex.IsMatch(str, @"w+([-+.']w+)*@w+([-.]w+)*.w+([-.]w+)*");
}

using System.Text.RegularExpressions;
if  (!Regex.IsMatch(txtEmail.Text, @"^[a-z,A-Z]{1,10}((-|.)w+)*@w+.w{3}$"))
        MessageBox.Show("Not valid email.");

string EmailPattern = @"w+([-+.']w+)*@w+([-.]w+)*.w+([-.]w+)*";
if (Regex.IsMatch(Email, EmailPattern, RegexOptions.IgnoreCase))
{
    Console.WriteLine("Email: {0} is valid.", Email);
}
else
{
    Console.WriteLine("Email: {0} is not valid.", Email);
}

string patternEmail = @"(?<email>w+@w+.[a-z]{0,3})";
Regex regexEmail = new Regex(patternEmail);

^[w!#$%&'*+-/=?^_`{|}~]+(.[w!#$%&'*+-/=?^_`{|}~]+)*@((([-w]+.)+[a-zA-Z]{2,4})|(([0-9]{1,3}.){3}[0-9]{1,3}))$

^(([^<>()[]\.,;:s@""]+(.[^<>()[]\.,;:s@""]+)*)|("".+""))@(([[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}])|(([a-zA-Z-0-9]+.)+[a-zA-Z]{2,}))$

public static bool IsValidEmail(string email)
    {
        var r = new Regex(@"^([0-9a-zA-Z]([-.w]*[0-9a-zA-Z])*@([0-9a-zA-Z][-w]*[0-9a-zA-Z].)+[a-zA-Z]{2,9})$");
        return !String.IsNullOrEmpty(email) && r.IsMatch(email);
    }

public static bool IsValidEmailAddress(this string emailaddress)
    {
        try
        {
            MailAddress m = new MailAddress(emailaddress);
            return true;
        }
        catch (FormatException)
        {
            return false;
        }
    }

string customerEmailAddress = "bert@potato.com";
customerEmailAddress.IsValidEmailAddress()