Connection limit reached in Exchange

Since a week ago i have a great problem with my mail server. Continuosly I obtain the next error in the clients (outlook express): "Error 421: Connection limit reached". I modified the values of connections inbound and outbound of the IMC, but this isn't the problem. I think that my server is being attack by anybody. I think that my server is open relay free. Anybody know as i can to find a solution to my problem. I'm not a great expert in network security.

Thanks in advance

Fernando Gonz?lez Mac?as
fgonzalez@diariodecadiz.com
Dpto. Informatica - Diario de C?diz (Spain)