EHLO being rejected but HELO works

[beddo]

Hi there,
I've been having a nightmare with one account on our server.
First off POP3 incoming was going wonky so I've had mail send to the end user via SMTP and set up hmailserver locally to deal with it.
Now all of a sudden outgoing mail has stopped working :/

Turns out that if I do a telnet test, the EHLO command is immediately rejected with a 502.
If I repeated with HELO it works fine but hmailserver cannot be told to use HELO instead of EHLO and there are also Outlook clients trying to send mail that are being blocked.

It only does this from the customer's IP as the same details work from elsewhere.

Anyone know why exim would reject EHLO but not HELO?

[beddo]

OK, the plot thickens.
We have changed the end users IP and the problem is still happening. It can't be a coincidence that the account is experiencing both inbound and outbound mail problems and yes I have completely deleted/readded the account to the server.

[beddo]

Little bit more, turns out I was testing from a machine with Norton IS on it (ick).

Disabled that and it gets more annoying.

I telnet on and get the welcome banner and then the moment I type anything, the connection drops. No 502, nothing it just resets the connection.