Cannot send attachments with emails after...

Question

I have just installed a NetCommV300 adaptor router with my existing NetComm NB5Plus4 modem. I now cannot send attachments with my emails. A message says,"Connection to mail server interupted" . NetComm advised me to play around with the MTU settings in themodem also the MTU enforcer but this does not fix the problem. Any ideas? Many thanks. Geoff.