Replace nested foreach to Increase performance

var usedLists = new HashSet<long>();
foreach (var test in Tests)
{
  var requiredLists = this.GetLists(test.test, selectedTag);

  foreach (var List in requiredLists)
  {   
    if (!usedLists.Contains(List.Id))
    {
      usedLists.Add(List.Id);
      var toRecipients = List.RecepientsTo;
      var ccRecipients = List.RecipientsCC;
      var bccRecipients = List.RecipientsBCC;
      var replyTo = new List<string>() { List.ReplyTo };
      var mailMode = isPreviewMode ? MailMode.Display : MailMode.Drafts;
      OutlookModel.Instance.CreateEmail(toRecipients, ccRecipients, bccRecipients, this.Draft, mailMode, replyTo);
    }
  }
}