MailMessage.BodyFormat генерирует неизвестную ошибку, когда она равна MailFormat.Html

Хотя уже есть несколько ответов, я подумал, что предлагаю немного альтернативный подход, используя Array.prototype.map(), завернутый в функцию, которая может быть адаптирована пользователем (для обновления значения n в символе nth и измените используемый символ замены):

// defining the named function, with an 'opts' argument:
function replaceNthWith(opts) {

  // setting the default options:
  var defaults = {

    // defining the nth character, in this case
    // every fourth:
    'nth': 4,

    // defining the character to replace that
    // nth character with:
    'char': '|'
  };

  // Note that there's no default string argument,
  // so that one argument must be provided in the
  // opts object.

  // iterating over each property in the
  // opts Object:
  for (var property in opts) {

    // if the current property is a property of
    // this Object, not inherited from the Object 
    // prototype:
    if (opts.hasOwnProperty(property)) {

      // we set that property of the defaults
      // Object to be equal to that property
      // as set in the opts Object:
      defaults[property] = opts[property];
    }
  }

  // if there is a defaults.string property
  // (inherited from the opts.string property)
  // then we go ahead; otherwise nothing happens
  // note: this property must be set for the
  // function to do anything useful:
  if (defaults.string) {

    // here we split the string supplied from the user,
    // via opts.string, in defaults.string to form an
    // Array of characters; we iterate over that Array
    // with Array.prototype.map(), which process one
    // Array and returns a new Array according to the
    // anonymous function supplied:
    return haystack = defaults.string.split('').map(function(character, index) {

      // here, when the index of the current letter in the
      // Array formed by Array.prototype.split() plus 1
      // (JavaScript is zero-based) divided by the number
      // held in defaults.nth is equal to zero - ensuring
      // that the current letter is the 'nth' index we return
      // the defaults.char character; otherwise we return
      // the original character from the Array over which
      // we're iterating:
      return (index + 1) % parseInt(defaults.nth) === 0 ? defaults.char : character;

    // here we join the Array back into a String, using
    // Array.prototype.join() with an empty string:
    }).join('');
  }

}

// 'snippet.log()' is used only in this demonstration, in real life use
// 'console.log()', or print to screen or display in whatever other
// method you prefer:
snippet.log( replaceNthWith({ 'string': "abcdefoihewfojias" }) );

function replaceNthWith(opts) {
  var defaults = {
    'nth': 4,
    'char': '|'
  };

  for (var property in opts) {
    if (opts.hasOwnProperty(property)) {
      defaults[property] = opts[property];
    }
  }

  if (defaults.string) {
    return haystack = defaults.string.split('').map(function(character, index) {

      return (index + 1) % parseInt(defaults.nth) === 0 ? defaults.char : character;

    }).join('');
  }

}

// 'snippet.log()' is used only in this demonstration, in real life use
// 'console.log()', or print to screen or display in whatever other
// method you prefer.

// calling the function, passing in the supplied 'string'
// property value:
snippet.log( replaceNthWith({
    'string': "abcdefoihewfojias"
}) );
// outputs: abc|efo|hew|oji|s

// calling the function with the same string, but to replace
// every second character ( 'nth' : 2 ):
snippet.log( replaceNthWith({
    'string': "abcdefoihewfojias",
    'nth': 2
}) );
// outputs: a|c|e|o|h|w|o|i|s

// passing in the same string once again, working on every
// third character, and replacing with a caret ('^'):
snippet.log( replaceNthWith({
    'string': "abcdefoihewfojias",
    'nth': 3,
    'char' : '^'
}) );
// outputs: ab^de^oi^ew^oj^as

Ссылки:

• Array.prototype.map() .
• Array.prototype.join() .
• for...in loop .
• Оператор останова JavaScript (%) .
• Object.prototype.hasOwnProperty() .
• String.prototype.split() .