The connect acl is called when the client first establishes a connection. At this point whe know the clients ip address but not much else. It is normal to do a DNS lookup at this time.

First we check that ip address against a white list. If found the address status is set to whiteListed and the address is accepted.

If not found on a white list, we check that ip address against black lists. First is our internal black list. If it is not found there we check against several blacklists on the web. If the IP is found on a black list the status is set to blackListed and the IP address is accepted.

If the IP is not found on either a white list or a black list, it's status is set to notListed and the IP address is accepted.

The helo acl runs when the helo command is given. It's job is to check some key elements of the calling client:

• 1 - did the client give us their host name or ip address
• 2 - is the client using a host name that matches dns records
• 3 - is the host name or ip address the same as ours ( a trick used by spammers )

If the client gave us our host name or IP address the connection is rejected

Otherwise, a marker is placed on the connection indicating if the client gave us a valid host name, an IP address in lieu of a host name or no host name at all. In any of these events the command is accepted.

The mail acl fires when a MAIL command is received i.e. MAIL FROM. This is who is sending the mail i.e. the senders address.

Since we are not a open relay we handle mail for two entities. Mail from our clients OR mail to our clients.

Therefore, we have to verify that either the senders address OR the recipients address resides within one of our domains.

Although we have the senders address we can not verify it in this acl because the acl does not support it.

The rcpt acl fires when the rcpt command is given. At this time we know both the sender (since that command has to come before this one) and the reciever. Both addresses are verified. At least one has to be within our domain structure.

The RCPT command can be given many times since an email can be sent to many recipients. In each case, the sending OR recieving address has to be within our domain.

If either of the addesses are within our domain, the RCPT address is accepted, otherwise it is rejected.

The data acl fires when the actual email message is received. At this pont we have accepted the addresses in whole or in part and we are processing the message.

Frist we need to ensure that the message has no viruses because that would be bad. If the message contains any viruses it will be automatically rejected and the connection will be aborted.

The second task is to get a spam score on the message. If the spam score is too high the message will be labeled as spam and will probably be put in a spam folder. However, regardless of the spam score the message will be accepted.

Returns accept if all conditions of the acl are met unless endpass was used. In the event endpass is used on a line by itself between to conditions/modifiers, if that preceeding endpass fails, control is passed to the next acl statement. If the condition after endpass fails, the acl returns deny.

If all conditions are met the acl returns defer (returns a 4xx code) otherwise, if any of the conditions are not met, control goes to the next acl statement.

Returns deny if all conditions are met otherwise, control passes to the next acl statement.

Same as deny except the connection is dropped after a permanent error code is sent.

Control is passed to the next acl statement if all conditions are met. Returns deny otherwise.

If all conditions are met actions of any modifiers are carried out and control passes to the next acl statment.

Adds a header to the message. This should be at the end of the acl statement after all conditions because it adds the header even if a later condition fails.

Causes the message to be delayed in processing. Great for spammers but it does tie up resources.

Only allowed in accept and discard statments. In normal processing of an acl statement, when a condition in an acl statement fails the processing continues with the next acl statement.

When endpass is used, if the next acl condition (that immediately following it) fails, the acl returns deny.

Example:

Normally, without endpass, if the second condition was unsuccessful control would be passed to the next acl statement. However, with endpass if the second condition is false, the acl statement returns deny.

Specifies an addtional log to write reject log entries to

Logs a message to the log if the conditions of the acl are all true

Like log_message but it logs even if the acl failed

Returns a message to the client.

Specifies the a queue to use. Must be named before the message is recieve. Not available in a data acl.

Removes and existing header(s). The parameter can be a colon separated list.

Sets the value of a user defined variable. The variable name must begin with acl_m_ or acl_c_. acl_c_ will be available throughout the session and can be used in routers and transports while acl_m_ is disposed after the message is received.

sends a udp packet

Allows another acl to be called from within an acl

A list of DNS black lists to check

Tests if the session is encrypted

Checks the sending host against a list

Checks the local_part against a list or string

Checks the recipient address against a list or a string

Checks the senders domain against a string or a list

Test the sender address against a string

Verifies the sender or recipient addres by running it through the routers