Error Message:

System.Net.Mail.SmtpException: Failure sending mail. ---> System.IndexOutOfRangeException: Index was outside the bounds of the array.

   at System.Net.Base64Stream.EncodeBytes(Byte[] buffer, Int32 offset, Int32 count, Boolean dontDeferFinalBytes, Boolean shouldAppendSpaceToCRLF)

   at System.Net.Base64Stream.Write(Byte[] buffer, Int32 offset, Int32 count)

   at System.Net.Mime.MimePart.Send(BaseWriter writer)

   at System.Net.Mime.MimeMultiPart.Send(BaseWriter writer)

   at System.Net.Mail.SmtpClient.Send(MailMessage message)

   --- End of inner exception stack trace ---

   at System.Net.Mail.SmtpClient.Send(MailMessage message)

   at Microsoft.Crm.Tools.Email.Providers.CrmPollingSendEmailProvider.Run()

Resolution:

1. Apply the patch (NDP40-KB2183292-x64.exe) for .Net Framework 4.
2. Reboot the Server if prompted.

Refer http://support.microsoft.com/kb/2183292 for more information.