To enable SMTP relay filtering, simply select the Enable Relay Filtering check box at the top of the Relay Filtering page. Once enabled, both the Allow and Restrict sections of the Relay Filtering dialog tab will become active. Those users with IP addresses that appear within the Restrict box will not be allowed to use your mail server as an SMTP relay. The Allow section has priority over the Restrict section, so if the same address is in both sections, that address will still be allowed to relay off your server.
If the Restrict All option within the Restrict section is selected, no users except those specifically set within the Allow section will be allowed to send email via your mail server.
The Allow and Restrict fields may only contain IP addresses or ranges of IP addresses. To add an IP address or range, just type it into the Allow or Restrict text boxes.
Sometimes your users need to send mail through your server (Relay) when they are not at a site which is selected in your Allow field. This can often be the case, for example, when traveling sales people use a national ISP while away from the office. Instead of having to allow any IP address from that ISP relay access through your server, you can instead setup Authenticated POP access.
This option will allow relay access for a specific IP address even if that IP address is restricted, if there has been a successful POP3 access from that IP address within a specified amount of time. The time frame can be specified anywhere from seconds to days.
Note: It is imperative that when you select the Restrict All option, you also either enter IP addresses of those who use your mail server to send mail in the Allow dialog box or select the Authenticated POP access option. If you do not do this, nobody will be able to send email from your server to remote sites!
When mail is rejected due to SMTP relay filtering, SLmail will make special note within its log files. The following are possible messages that may appear when mail is rejected due to relay filtering.
If a relay filter matches, the following is logged within the slsmtp.evn log file under event 256:
YY/MM/DD HH:MM:SS TRANSACTION#: SMTP Relay denied for [Connecting IP] to otherdomain. (FILTER THAT MATCHED)