That is because Norton does not play very nice with any email client. With Norton you will need to DISABLE the automatic incoming and outgoing email scanner. If disabling does not work then you will need to uninstall Norton as per their uninstall tool (available from the Symantec site) then re-install it without the email scanning feature.
Better still install something like free Avast,AVG,or Nod 32 but without the email scanning feature. You will still be protected with the resident AV if you try to open any infector in your email(s)