我总结了以上所有的答案，截至2021年，我在这门课上为自己写的答案:

public static class StringExt {
    private const string emailPattern = @"^(?!\.)(""([^""\r\\]|\\[""\r\\])*""|" 
            + @"([-a-z0-9!#$%&'*+/=?^_`{|}~]|(?<!\.)\.)*)(?<!\.)" 
            + @"@[a-z0-9][\w\.-]*[a-z0-9]\.[a-z][a-z\.]*[a-z]$";

    public static bool IsValidMailAddress(this string pThis) 
            => pThis is not null 
            && Regex.IsMatch(pThis, emailPattern, RegexOptions.IgnoreCase);
}

For the simple email like goerge@xxx.com, below code is sufficient. 

 public static bool ValidateEmail(string email)
        {
            System.Text.RegularExpressions.Regex emailRegex = new System.Text.RegularExpressions.Regex(@"^([\w\.\-]+)@([\w\-]+)((\.(\w){2,3})+)$");
            System.Text.RegularExpressions.Match emailMatch = emailRegex.Match(email);
            return emailMatch.Success;
        }

There are a lot of strong answers here. However, I recommend that we take a step back. @Cogwheel answers the question https://stackoverflow.com/a/1374644/388267. Nevertheless, it could be costly in a bulk validation scenario, if many of the email address being validated are invalid. I suggest that we employ a bit of logic before we enter into his try-catch block. I know that the following code could be written using RegEx but that could be costly for new developers to understand. This is my twopence worth:

    public static bool IsEmail(this string input)
    {
        if (string.IsNullOrWhiteSpace(input)) return false;

        // MUST CONTAIN ONE AND ONLY ONE @
        var atCount = input.Count(c => c == '@');
        if (atCount != 1) return false;

        // MUST CONTAIN PERIOD
        if (!input.Contains(".")) return false;

        // @ MUST OCCUR BEFORE LAST PERIOD
        var indexOfAt = input.IndexOf("@", StringComparison.Ordinal);
        var lastIndexOfPeriod = input.LastIndexOf(".", StringComparison.Ordinal);
        var atBeforeLastPeriod = lastIndexOfPeriod > indexOfAt;
        if (!atBeforeLastPeriod) return false;

        // CODE FROM COGWHEEL'S ANSWER: https://stackoverflow.com/a/1374644/388267 
        try
        {
            var addr = new System.Net.Mail.MailAddress(input);
            return addr.Address == input;
        }
        catch
        {
            return false;
        }
    }

.net 4.5增加了System.ComponentModel.DataAnnotations.EmailAddressAttribute

你可以浏览EmailAddressAttribute的源代码，这是它内部使用的正则表达式:

const string pattern = @"^((([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+(\.([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+)*)|((\x22)((((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(([\x01-\x08\x0b\x0c\x0e-\x1f\x7f]|\x21|[\x23-\x5b]|[\x5d-\x7e]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(\\([\x01-\x09\x0b\x0c\x0d-\x7f]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF]))))*(((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(\x22)))@((([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.)+(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.?$";

识别电子邮件id是否有效的简单方法。

public static bool EmailIsValid(string email)
{
        return Regex.IsMatch(email, @"^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$");
}

正如在许多回答中提到的，电子邮件地址的领域是复杂的。在这种情况下，我强烈反对使用正则表达式。那些匹配(大多数)用例的异常复杂，难以读取，因此也难以维护。此外，它们仍然难以支持所有病例，而且速度缓慢。

微软的EmailAddress类在这方面有所帮助，但我认为也不是完美的。在几年前的一个开源项目中，我尝试使用定制的EmailParser。

在[EmailAddress]https://github.com/Qowaiv/Qowaiv/blob/master/src/Qowaiv/EmailAddress.cs中使用。

通过使用这种方法，不仅可以验证电子邮件地址，还可以清除显示名称的多种格式，去掉mailto:-前缀，并根据ip地址规范化域字面量，并将所有内容小写(注意，本地部分是大小写敏感的)。

你的解决方案应该支持的场景(上面提到的就是):

[TestCase(null)]
[TestCase("")]
[TestCase("..@test.com")]
[TestCase(".a@test.com")]
[TestCase("ab@sd@dd")]
[TestCase(".@s.dd")]
[TestCase("ab@988.120.150.10")]
[TestCase("ab@120.256.256.120")]
[TestCase("ab@120.25.1111.120")]
[TestCase("ab@[188.120.150.10")]
[TestCase("ab@188.120.150.10]")]
[TestCase("ab@[188.120.150.10].com")]
[TestCase("a@b.-de.cc")]
[TestCase("a@bde-.cc")]
[TestCase("a@bde.c-c")]
[TestCase("a@bde.cc.")]
[TestCase("ab@b+de.cc")]
[TestCase("a..b@bde.cc")]
[TestCase("_@bde.cc,")]
[TestCase("plainaddress")]
[TestCase("plain.address")]
[TestCase("@%^%#$@#$@#.com")]
[TestCase("@domain.com")]
[TestCase("Joe Smith <email@domain.com>")]
[TestCase("email.domain.com")]
[TestCase("email@domain@domain.com")]
[TestCase(".email@domain.com")]
[TestCase("email.@domain.com")]
[TestCase("email..email@domain.com")]
[TestCase("email@-domain.com")]
[TestCase("email@domain-.com")]
[TestCase("email@domain.com-")]
[TestCase("email@.domain.com")]
[TestCase("email@domain.com.")]
[TestCase("email@domain..com")]
[TestCase("email@111.222.333")]
[TestCase("email@111.222.333.256")]
[TestCase("email@[123.123.123.123")]
[TestCase("email@[123.123.123].123")]
[TestCase("email@123.123.123.123]")]
[TestCase("email@123.123.[123.123]")]
[TestCase("email@{leftbracket.com")]
[TestCase("email@rightbracket}.com")]
[TestCase("email@p|pe.com")]
[TestCase("isis@100%.nl")]
[TestCase("email@dollar$.com")]
[TestCase("email@r&d.com")]
[TestCase("email@#hash.com")]
[TestCase("email@wave~tilde.com")]
[TestCase("email@exclamation!mark.com")]
[TestCase("email@question?mark.com")]
[TestCase("email@obelix*asterisk.com")]
[TestCase("email@grave`accent.com")]
[TestCase("email@colon:colon.com")]
[TestCase("email@caret^xor.com")]
[TestCase("email@=qowaiv.com")]
[TestCase("email@plus+.com")]
[TestCase("email@domain.com>")]
[TestCase("email( (nested) )@plus.com")]
[TestCase("email)mirror(@plus.com")]
[TestCase("email@plus.com (not closed comment")]
[TestCase("email(with @ in comment)plus.com")]
[TestCase(@"""Joe Smith email@domain.com")]
[TestCase(@"""Joe Smith' email@domain.com")]
[TestCase(@"""Joe Smith""email@domain.com")]
[TestCase("email@mailto:domain.com")]
[TestCase("mailto:mailto:email@domain.com")]
[TestCase("Display Name <email@plus.com> (after name with display)")]
[TestCase("ReDoSaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa@aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")]
public void IsInvalid(string email)
{
    Assert.IsFalse(EmailAddress.IsValid(email), email);
}

[TestCase("w@com")]
[TestCase("w.b.f@test.com")]
[TestCase("w.b.f@test.museum")]
[TestCase("a.a@test.com")]
[TestCase("ab@288.120.150.10.com")]
[TestCase("ab@188.120.150.10")]
[TestCase("ab@1.0.0.10")]
[TestCase("ab@120.25.254.120")]
[TestCase("ab@01.120.150.1")]
[TestCase("ab@88.120.150.021")]
[TestCase("ab@88.120.150.01")]
[TestCase("ab@[120.254.254.120]")]
[TestCase("local@2001:0db8:85a3:0000:0000:8a2e:0370:7334")]
[TestCase("local@[2001:0db8:85a3:0000:0000:8a2e:0370:7334]")]
[TestCase("2@bde.cc")]
[TestCase("-@bde.cc")]
[TestCase("a2@bde.cc")]
[TestCase("a-b@bde.cc")]
[TestCase("ab@b-de.cc")]
[TestCase("a+b@bde.cc")]
[TestCase("f.f.f@bde.cc")]
[TestCase("ab_c@bde.cc")]
[TestCase("_-_@bde.cc")]
[TestCase("k.haak@12move.nl")]
[TestCase("K.HAAK@12MOVE.NL")]
[TestCase("email@domain.com")]
[TestCase("email@domain")]
[TestCase("あいうえお@domain.com")]
[TestCase("local@あいうえお.com")]
[TestCase("firstname.lastname@domain.com")]
[TestCase("email@subdomain.domain.com")]
[TestCase("firstname+lastname@domain.com")]
[TestCase("email@123.123.123.123")]
[TestCase("email@[123.123.123.123]")]
[TestCase("1234567890@domain.com")]
[TestCase("a@domain.com")]
[TestCase("a.b.c.d@domain.com")]
[TestCase("aap.123.noot.mies@domain.com")]
[TestCase("1@domain.com")]
[TestCase("email@domain-one.com")]
[TestCase("_______@domain.com")]
[TestCase("email@domain.topleveldomain")]
[TestCase("email@domain.co.jp")]
[TestCase("firstname-lastname@domain.com")]
[TestCase("firstname-lastname@d.com")]
[TestCase("FIRSTNAME-LASTNAME@d--n.com")]
[TestCase("first-name-last-name@d-a-n.com")]
[TestCase("{local{name{{with{@leftbracket.com")]
[TestCase("}local}name}}with{@rightbracket.com")]
[TestCase("|local||name|with|@pipe.com")]
[TestCase("%local%%name%with%@percentage.com")]
[TestCase("$local$$name$with$@dollar.com")]
[TestCase("&local&&name&with&$@amp.com")]
[TestCase("#local##name#with#@hash.com")]
[TestCase("~local~~name~with~@tilde.com")]
[TestCase("!local!!name!with!@exclamation.com")]
[TestCase("?local??name?with?@question.com")]
[TestCase("*local**name*with*@asterisk.com")]
[TestCase("`local``name`with`@grave-accent.com")]
[TestCase("^local^^name^with^@xor.com")]
[TestCase("=local==name=with=@equality.com")]
[TestCase("+local++name+with+@equality.com")]
[TestCase("Joe Smith <email@domain.com>")]
[TestCase("email@domain.com (joe Smith)")]
[TestCase(@"""Joe Smith"" email@domain.com")]
[TestCase(@"""Joe\\tSmith"" email@domain.com")]
[TestCase(@"""Joe\""Smith"" email@domain.com")]
[TestCase(@"Test |<gaaf <email@domain.com>")]
[TestCase("MailTo:casesensitve@domain.com")]
[TestCase("mailto:email@domain.com")]
[TestCase("Joe Smith <mailto:email@domain.com>")]
[TestCase("Joe Smith <mailto:email(with comment)@domain.com>")]
[TestCase(@"""With extra < within quotes"" Display Name<email@domain.com>")]
[TestCase("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa@aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")]
public void IsValid(string email)
{
    Assert.IsTrue(EmailAddress.IsValid(email), email);
}