We have enabled an IOS based IPS on our internet routers and all seems to be working well apart from a strage issue. We are currently only alerting on all signitures so no traffic should be droppped. The problem i have is with incoming hotmail and yahoo mail messages which fail to be delivered to the recipents on our mail server. When we disable the IPS the mails soon arrives after.
I have applied the IPS inbound on the internet faceing interface. Can anyone shed some light as to why we have this issue.