You could use the Outlook Redemption library which is capable of opening PST and extracting messages as .EML (among other formats). Redemption is a COM Object (32 or 64 bit) that can be used in C# without any problem. Here is a Console Application sample code that demonstrates this:

using System;
using System.IO;
using System.Text;
using Redemption;

namespace DumpPst
{
    class Program
    {
        static void Main(string[] args)
        {
            // extract 'test.pst' in the 'test' folder
            ExtractPst("test.pst", Path.GetFullPath("test"));
        }

        public static void ExtractPst(string pstFilePath, string folderPath)
        {
            if (pstFilePath == null)
                throw new ArgumentNullException("pstFilePath");

            RDOSession session = new RDOSession();
            RDOPstStore store = session.LogonPstStore(pstFilePath);
            ExtractPstFolder(store.RootFolder, folderPath);
        }

        public static void ExtractPstFolder(RDOFolder folder, string folderPath)
        {
            if (folder == null)
                throw new ArgumentNullException("folder");

            if (folderPath == null)
                throw new ArgumentNullException("folderPath");

            if (folder.FolderKind == rdoFolderKind.fkSearch)
                return;

            if (!Directory.Exists(folderPath))
            {
                Directory.CreateDirectory(folderPath);
            }

            foreach(RDOFolder child in folder.Folders)
            {
                ExtractPstFolder(child, Path.Combine(folderPath, ToFileName(child.Name)));
            }

            foreach (var item in folder.Items)
            {
                RDOMail mail = item as RDOMail;
                if (mail == null)
                    continue;

                mail.SaveAs(Path.Combine(folderPath, ToFileName(mail.Subject)) + ".eml", rdoSaveAsType.olRFC822);
            }
        }

      /// <summary>
      /// Converts a text into a valid file name.
      /// </summary>
      /// <param name="fileName">The file name.</param>
      /// <returns>
      /// A valid file name.
      /// </returns>
      public static string ToFileName(string fileName)
      {
          return ToFileName(fileName, null, null);
      }

      /// <summary>
      /// Converts a text into a valid file name.
      /// </summary>
      /// <param name="fileName">The file name.</param>
      /// <param name="reservedNameFormat">The reserved format to use for reserved names. If null '_{0}_' will be used.</param>
      /// <param name="reservedCharFormat">The reserved format to use for reserved characters. If null '_x{0}_' will be used.</param>
      /// <returns>
      /// A valid file name.
      /// </returns>
      public static string ToFileName(string fileName, string reservedNameFormat, string reservedCharFormat)
      {
          if (fileName == null)
              throw new ArgumentNullException("fileName");

          if (string.IsNullOrEmpty(reservedNameFormat))
          {
              reservedNameFormat = "_{0}_";
          }

          if (string.IsNullOrEmpty(reservedCharFormat))
          {
              reservedCharFormat = "_x{0}_";
          }

          if (Array.IndexOf(ReservedFileNames, fileName.ToLowerInvariant()) >= 0 ||
              IsAllDots(fileName))
              return string.Format(reservedNameFormat, fileName);

          char[] invalid = Path.GetInvalidFileNameChars();

          StringBuilder sb = new StringBuilder(fileName.Length);
          foreach (char c in fileName)
          {
              if (Array.IndexOf(invalid, c) >= 0)
              {
                  sb.AppendFormat(reservedCharFormat, (short)c);
              }
              else
              {
                  sb.Append(c);
              }
          }

          string s = sb.ToString();

          // directory limit is 255
          if (s.Length > 254)
          {
              s = s.Substring(0, 254);
          }

          if (string.Equals(s, fileName, StringComparison.Ordinal))
          {
              s = fileName;
          }
          return s;
      }

      private static bool IsAllDots(string fileName)
      {
          foreach (char c in fileName)
          {
              if (c != '.')
                  return false;
          }
          return true;
      }

      private static readonly string[] ReservedFileNames = new[]
      {
          "con", "prn", "aux", "nul",
          "com0", "com1", "com2", "com3", "com4", "com5", "com6", "com7", "com8", "com9",
          "lpt0", "lpt1", "lpt2", "lpt3", "lpt4", "lpt5", "lpt6", "lpt7", "lpt8", "lpt9"
      };
    }
}