Exim - Max Connections Per IP?

Hi Everyone,

I have a dedicated server that I use to provide shared hosting for clients and today one of them got the following error repeatedly/relentlessly: 421 Too Many Concurrent SMTP Connections.

The host I use has set exim to allow a maximum of 5 concurrent connections per IP by default. I am thinking of increasing this to 15 or more but I don’t know if it should be even more or less?

Can any give me any pointers as to what it should be set at. Also, how would I work around this if I landed a client who had 100+ employees with email accounts being used from one IP?

As always, your input would be greatly appreciated.