If you use domain filtering, we recommend you to only allow Spamdrain's servers to connect to port 25 on your mail server.
This means that only the following IP addresses should be allowed to connect to TCP port 25 on your mail server:
62.75.203.192 - 62.75.203.223 (62.75.203.192/27)
85.25.218.200
85.25.217.33
85.25.217.35
85.25.217.7
85.25.43.228
85.25.74.85
85.25.207.204
85.25.207.221
85.25.207.243
85.25.217.223
85.25.237.107
188.138.41.142
188.138.102.7
188.138.41.110
188.138.75.226