[PERDITION-USERS] Fatal error writing to client

One of our client cannot connect to our incoming mail server. I have notice that their ip was having an error on the log Nov 16 09:21:53 perdition[26724]: Fatal error writing to client 1.2.3.4->10.20.30.40.Exiting child. Nov 16 09:51:56 perdition[31690]: Connect: 1.2.3.4->10.20.30.40 Nov 16 09:51:58 perdition[31690]: Fatal error writing to client 1.2.3.4->10.20.30.40.Exiting child. Nov 16 10:06:24 perdition[1673]: Connect: 1.2.3.4->10.20.30.40 Nov 16 10:06:24 perdition[1673]: Fatal error writing to client 1.2.3.4->10.20.30.40.Exiting child. Nov 16 10:08:25 perdition[1991]: Connect: 1.2.3.4->10.20.30.40 Nov 16 10:08:26 perdition[1991]: Fatal error writing to client 1.2.3.4->10.20.30.40.Exiting child. This is the only client who is experiencing this problem. Anyone can give a hint about this problem and any possible solution?