Added 4 useful methods

[XiXiongMaoXiong]

/**
* Returns the domain part of a given Email address
*
* @param email The Email address. E.g Returns "protonmail.com" from the given Email "johnsmith@protonmail.com".
* @return the domain part of a given Email address.
*/
public static String extractEmailProvider(String email) {
return email.substring(email.lastIndexOf("@") + 1);
}

/**
 * Returns the username part of a given Email address. E.g. Returns "johnsmith" from the given Email "johnsmith@protonmail.com".
 *
 * @param email The Email address.
 * @return the username part of a given Email address.
 */
public static String extractEmailUsername(String email) {
    return email.substring(0, email.lastIndexOf("@"));
}


/**
 * Return whether a given Email address is on a specified Email provider. E.g. "johnsmith@protonmail.com" and "gmail.com" will return false.
 *
 * @param email The Email address.
 * @param emailProvider The Email provider to testify against.
 * @return {@code true}: yes<br>{@code false}: no
 */
public static boolean isFromEmailProvider(String email, String emailProvider) {
    return extractEmailProvider(email).equalsIgnoreCase(emailProvider);
}

/**
 * Return whether a given Email address is on any of the specified Email providers list (array). E.g. Useful if you pass it a list of real Email provider services and check if the Email is a disposable Email or a real one.
 *
 * @param email The Email address.
 * @param emailProviders The list of Email providers to testify against.
 * @return {@code true}: yes<br>{@code false}: no
 */
public static boolean isFromAnyOfEmailProviders(String email, String[] emailProviders) {
    return com.blankj.utilcode.util.ArrayUtils.contains(emailProviders, extractEmailProvider(email));
}

[XiXiongMaoXiong]

@Blankj