The IP_ACCESS Mapping Table can be used to do a last moment check on the IP address to which the MTA is about to connect; the connection attempt can then be aborted or redirected. This can be useful under certain special circumstances, for example, security concerns about a destination IP address to which should never be connected, or where it is wished to avoid connecting to known-to-be-bogus destination IP addresses (for example, 127.0.0.1), or where you wish to attempt to fail over to another destination IP address similar to a lastresort keyword effect (see 12.4.3.7 Last Resort Host).
This access mapping is consulted during SMTP client operations just prior to attempting to open connections to a remote server. The mapping probe has the following format:
source-channel|address-current|address-count|ip-current|hostname |
source-channel is the channel from which the message is being dequeued. address-count is the total number of IP addresses for the remote server. address-current is the index of the current IP address being tried. ip-current is the current IP address. hostname is the symbolic name of the remote server. The table below shows the flags for this table.
Table 18–4 IP_ACCESS Mapping Table Flags
Flag |
Description |
---|---|
$N |
Immediately reject the message with an "invalid host/domain error." Any supplied text will be logged as the reason for rejection but will not be included in the DSN. |
$I |
Skip the current IP without attempting to connect. |
$A |
Replace the current IP address with the mapping result. |