[exim] too many connections

Hi,
I have an exim mail server with the max number of connections set to 150.
It has been running fine but over the last week or so, I am getting
large numbers of established smtp connections to my machine even when
the mail queue is completely clear.
Because of this I am ending up dropping connections. I would rather not
increase the limit because that real problem is the large number of
connections and not the current limit.

Could this be some sort of attempted dos attack?
Regards,

--
Michael