Message submission rate through SMTP Relay

Hi,

We have a IIS SMTP Relay setup on a 2008 R2 server running through to the O365 servers.

I am aware of the 30 emails per minute and 10,000 emails per day but we are exceeding the 30 mail per minute to a single account. and receiving the following:

Message submission rate for this client has exceeded the configured limit

I was wondering how to set the limit in the IIS SMTP relay to only send a maximum of 30 emails per minute so that we will not have this problem?

Thanks