[Mimedefang] filter_recipient questions

[Jeff Rife]

On 28 Apr 2006 at 9:18, Matthew.van.Eerde at hbinc.com wrote:

> Dirk the Daring wrote:
> >    What I'd like to add is something to check validity of local
> > recipients
> 
> Export the list of valid email addresses from your destination MTA
> periodically, build an access map, and sendmail takes care of
> rejecting invalid recipients before MIMEDefang is called. 

Actually, no it doesn't.  Invalid recipients *still* get passed on to 
the milter (maybe only if you have "delay_checks" enabled, but you need 
this to allow AUTH to be used from otherwise blacklisted IPs, so most 
people seem to have it set up).

Sendmail will do the right thing and reject a bad address at the SMTP 
phase if your milter doesn't do it, but that only means that the 
address isn't:

- a local user (as defined by whatever PAM mech you use)
- the left-hand side of an alias
- the left-hand side of a virtusertable entry

Sendmail doesn't do any more checking to see if the address is valid.  
Thus, if something ends up aliasing to something not deliverable, 
sendmail eventually has to send a bounce e-mail (instead of a reject 
during the SMPT conversation).  This is what the OP wanted to avoid,  
and I suspect why he said "other than using 
md_check_against_mx_server", since that uses identical logic if the 
endpoint server is running sendmail.  The logic is similar if the 
server is running Exchange, too...it only checks to see if the address 
is pointing to a mail-enabled object.  That object can then forward it 
to someplace non-deliverable, but Exchange doesn't check this until it 
is too late to reject at SMTP.


--
Jeff Rife | "Oooh, I love children... 
          |  they taste like chicken." 
          |  
          |         -- Heddy Newman, "Herman's Head"