I would like to propose a change on how "rcpt to" addresses are  
validated and messages are handled (expanded) and how queue modules  
know it needs to process this message.

I would like to change rcpt_to in smtpd_trans_rec to apr_table_t.  The  
key for the table will be the address sent to "rcpt to" command and the  
value will be a pointer to a apr_array_header_t list of pointers to the  
following structure:

typedef struct smtpd_rctp_rec {
	apr_pool_t *p;
	const char *mode;
	int validated;
	const char *address;
	apr_table_t info;
	const char *expanded;
}

mode: what queue handler should handle this message.
	Example would be local, relay, relay_to, forward, postfix.

validated: if the rctp has been validated and ready to process

address: what address to send to

info: extra data is stored that the queue handler might needed

expanded: log of how the address was expanded

mod_smtpd would create a hook for expanding rctp address which other  
modules will use to enable different ways to validate rcpt address and  
setting what queue module should process a message.  It would take a  
smtpd_rctp_rec * and a apr_array_header_t * of things to add to add and  
return decline, ok or delete.

HANDLER_DECLARE(rcpt) will create a smtpd_rctp_rec and run the  
expanding hook and handle the return value and also the things to be  
add.

Here are a few example of how I think it would work. This will be using  
some modules that do not exists yet and some that do:

mod_smtpd_user_pwd - handle validating local users and setting info on  
them

mod_smtpd_alias - enables aliases (forwarding email)

mod_smtpd_queue_local - handles the "local" mode
			Handles deliver of mail to local users

mod_smtpd_queue_relay - handles the "relay" and "forward" modes
			This module would hook the expanding hook using
			APR_HOOK_LAST since it will allow forwarding and
			relaying for authenticated user, but wants to make
			sure all other modules have a chance to change the
			address if needed.

mod_smtpd_queue_smtp - handles the "relay_to" mode
			Sends all mail to another server.
			This module would hook the expanding hook using
			APR_HOOK_FIRST since it want so to set every
			smtpd_rctp_rec mode to replay_to and doesn't
			care what other modules have to say.

Examples:

________________________________________________________________________ 
________

user@domain.tld -> user

expand hook:
	{ mode: NULL
	  address: "user@domain.tld"
	  validate: 0
	  info: { }
	  expanded: NULL
	}

mod_smtpd_user_pwd handles request and returns ok.
It changes the struct to look like this:

	{ mode: "local"
	  address: "user@domain.tld"
	  info: { "mail_box" -> "username" }
	  validate: 1
	  expanded: "user@domain.tld -> local user username"
	}

mod_smtpd_queue_local would handle queuing

________________________________________________________________________ 
________

auth'ed user relaying mail to user@otherdomain.tld

expand hook:
	{ mode: NULL
	  address: "user@otherdomain.tld"
	  validate: 0
	  info: { }
	  expanded: NULL
	}

mod_smtpd_auth will handle this and returns ok.
It changes the struct to look like this:

	{ mode: "relay"
	  address: "user@otherdomain.tld"
	  info: { }
	  validate: 1
	  expanded: "authenticated user <user> relaying user@otherdomain.tld"
	}

mod_smtpd_queue_relay would handle queuing

________________________________________________________________________ 
________

user@domain.tld -> user@offsite.tld

expand hook:
	{ mode: NULL
	  address: "user@domain.tld"
	  info: { }
	  validate: 0
	  expanded: NULL
	}

mod_smtpd_alias would handle the request and return delete.
It adds a new entry to the list that looks like this:

	{ mode: "forward"
	  address: "user@offsite.tld"
	  info: { }
	  validate: 0
	  expanded : "user@domain.tld -> user@offsite.tld"
	}

expand hook call on the new entry:

	{ mode: "forward"
	  address: "user@offsite.tld"
	  info: { }
	  validate: 0
	  expanded : "user@domain.tld -> user@offsite.tld"
	}

mod_smtpd_forward would handle the request and return ok.
It would set the struct like this:

	{ mode: "forward"
	  address: "user@offsite.tld"
	  info: { }
	  validate: 1
	  expanded : "user@domain.tld -> user@offsite.tld"
	}

mod_smtpd_queue_relay would handle queuing

________________________________________________________________________ 
________

info@company.tld
	-> user1@company.tld
		-> user11@offsite.tld
	-> user2@company.tld
		-> local user user2
	-> user3@company.tld
		-> user5@company.tld
			-> user55@offsite.tld

expand hook:
	{ mode: NULL
	  address: "info@company.tld"
	  info: { }
	  validate: 0
	  expanded: NULL
	}


mod_smtpd_alias would handle the request and return delete.
It would add new entries to the list that looked like this:

	{ mode: "forward"
	  address: "user11@offsite.tld"
	  info: { }
	  validate: 0
	  expanded: "info@company.tld -> user11@offsite.tld"
	}
	{ mode: "forward"
	  address: "user2@company.tld"
	  info { }
	  validate: 0
	  expanded: "info@company.tld -> user2@company.tld"
	}
	{ mode: "forward"
	  address: "user3@company.tld"
	  info { }
	  validate: 0
	  expanded: "info@company.tld -> user3@company.tld"
	}

expand hook on first new item:
	{ mode: "forward"
	  address: "user11@offsite.tld"
	  info: { }
	  validate: 0
	  expanded: "info@company.tld -> user11@offsite.tld"
	}

mod_smtpd_forward would handle the request and return ok
It would set the struct like this:

	{ mode: "forward"
	  address: "user11@offsite.tld"
	  info: { }
	  validate: 1
	  expanded: "info@company.tld -> user11@offsite.tld"
	}

expand hook on second new item:
	{ mode: "forward"
	  address: "user2@company.tld"
	  info { }
	  validate: 0
	  expanded: "info@company.tld -> user2@company.tld"
	}

mod_smtpd_user_pwd handles request and returns ok.
It set the struct to look like this:
	{ mode: "local"
	  address: "user2@company.tld"
	  info { "mail_box" -> "user2" }
	  validate: 1
	  expanded : "info@company.tld -> user2@company.tld -> local user  
user2"
	}

expand hook on third new item:
	{ mode: "forward"
	  address: "user3@company.tld"
	  info { }
	  validate: 0
	  expanded: "info@company.tld -> user3@company.tld"
	}

mod_smtpd_alias would handle the request and return delete.
It would add a new entry to the list that looked like this:

add list:
	{ mode: "forward"
	  address: "user5@company.tld"
	  info: { }
	  validate: 0
	  expanded: "info@company.tld -> user3@company.tld ->  
user5@company.tld"
	}

expand hook on third new new item:
	{ mode: "forward"
	  address: "user5@company.tld"
	  info: { }
	  validate: 0
	  expanded: "info@company.tld -> user3@company.tld ->  
user5@company.tld"
	}

mod_smtpd_alias would handle the request and return delete.
It would add a new entry to the list that looked like this:
	{ mode: "forward"
	  address: "user55@offsite.tld"
	  info: { }
	  validate: 0
	  expanded: "info@company.tld -> user3@company.tld ->  
user5@company.tld -> user55@offsite.tld"
	}

expand hook on third new new new item:
	{ mode: "forward"
	  address: "user55@offsite.tld"
	  info: { }
	  validate: 0
	  expanded: "info@company.tld -> user3@company.tld ->  
user5@company.tld -> user55@offsite.tld"
	}

mod_smtpd_forward would handle the request and return ok
It would set the struct like this:
	{ mode: "forward"
	  address: "user55@offsite.tld"
	  info: { }
	  validate: 1
	  expanded: "info@company.tld -> user3@company.tld ->  
user5@company.tld -> user55@offsite.tld"
	}

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