This one is more or less there.
Before I programmed the sample Code, I got the same Result with only a lenght of 1300 characters ... still a little bit long.

You might want to add something like this:
if ( ( laenge_eingabe_string < 6 ) || ( laenge_eingabe_string > 254 ) )
{
return 12; // Laenge: Laengenbegrenzungen stimmen nicht
}

private static EmailValidator m_email4j_v = new EmailValidator();

public static void main( String[] args )
{
try
{
String z = "ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ";

  String my_z_email_adress = FkString.left( z + z + z + z + z + z + z + z + z + z + z + z + z + z + z, 1000 ) + "@could.be.tolong.who.cares.info";

  System.out.println( "Length " + my_z_email_adress.length() );
  System.out.println( "Test 1 " + m_email4j_v.isValid( my_z_email_adress ) );

  my_z_email_adress = FkString.left( z + z + z + z + z + z + z + z + z + z + z + z + z +  z + z + z + z + z + z + z + z + z + z + z + z + z + z + z , 1500 ) + "@could.be.tolong.who.cares.info";
  
  System.out.println( "Length " + my_z_email_adress.length() );
  System.out.println( "Test 2 " + m_email4j_v.isValid( my_z_email_adress ) );

  my_z_email_adress = FkString.left( z + z + z + z + z + z + z + z + z + z + z + z + z +  z + z + z + z + z + z + z + z + z + z + z + z + z + z + z , 2000 ) + "@could.be.tolong.who.cares.info";

  System.out.println( "Length " + my_z_email_adress.length() );
  System.out.println( "Test 3 " + m_email4j_v.isValid( my_z_email_adress ) );
}
catch ( Exception err_inst )
{
  System.out.println( "MyTest"  + err_inst.getMessage() );
}

System.exit( 0 );

}

Results in:
Length 1031
Test 1 true
Length 1531
Test 2 true
Length 2031
Exception in thread "main" java.lang.StackOverflowError
at java.base/java.lang.Character.codePointAt(Character.java:8265)
at java.base/java.util.regex.Pattern$CharProperty.match(Pattern.java:3925)
at java.base/java.util.regex.Pattern$Branch.match(Pattern.java:4736)
at java.base/java.util.regex.Pattern$GroupHead.match(Pattern.java:4791)
at java.base/java.util.regex.Pattern$Loop.match(Pattern.java:4928)
at java.base/java.util.regex.Pattern$GroupTail.match(Pattern.java:4850)
at java.base/java.util.regex.Pattern$BranchConn.match(Pattern.java:4700)
at java.base/java.util.regex.Pattern$CharProperty.match(Pattern.java:3927)
at java.base/java.util.regex.Pattern$Branch.match(Pattern.java:4736)
at java.base/java.util.regex.Pattern$GroupHead.match(Pattern.java:4791)
at java.base/java.util.regex.Pattern$Loop.match(Pattern.java:4928)