Trouble with addresses using BCC

I often get mail I want to send on using BCC , sometimes I can list all the
addresses I want to send the email to, othertimes can do say 5 & then it
won't copy any more, other times it works ok, sometimes just won't copy
any at all. I do have all the addresses I want to use stored in Contacts.

I do not have that problem with my Yahoo a/c or my WLM a/c, only WM. Any
idea what is happening