mx records directing to intermediate servers

[Barry Margolin]

In article <cdjmi7$23kh$1 at sf1.isc.org>, lililevy at zahav.net.il (lili) 
wrote:

> hi, 
> 
> i have a basic question on mail transfer:
> 
> when i send an email, 
> my smtp server looks for the recipient smtp server in the dns (mx
> record).
> when will the mx record direct the emil to an intermediate smpt
> server,
> and when will it direct the email to the final smtp server (=so the
> email will be transferred directly to it's destination, with no
> intermediate stations).
> 
> are they any rules? standards? common use? 

See RFC 2821 for the full details.  Basically, it tries the servers in 
increasing order of the preference value (and servers with the same 
value are tried in random order among themselves) until it is successful 
at making an SMTP connection.  Some mailers will also fail over to a 
backup server if the connection is made but there's a fatal error during 
the SMTP dialogue; however, I don't think this is common, so you 
shouldn't depend on it.  In particular, you shouldn't have different 
communities of users on each server, because the sending server will not 
fail over when it gets an "Unknown recipient" error -- as soon as it 
gets this error from any receiver it will return the message to the 
sending user.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***