Bounced Mail - DNS Problems?

[Johnny Stork]

I believe I have my mail servers and/or DNS misconfigured and a bounced 
messages today has forced the issue to where I think I need to resolve. 
Let me first try to explain the network.

Primary SMTP Server (gateway.johnnystork.ca) - Only an internal ip 
(192.168.1.2) - Runs Mailscanner and Sendmail
An a-record exists as smtp.johnnystork.ca (so I figure this is where all 
outgoing messages should appear to be coming from)

Since Sendmail is not running on gateway and I cant use the "Masquerade 
as smtp.johnnystork" setting (which runs Mailscanner), how can I set 
thing up so ALL outgoing mail appears to come from smtp.johnnystork.ca? 
Most messages appear to show up as coming from gateway.johnnystork.ca. I 
also need to retain the internal hostname of gateway.johnnystork.ca for 
other reasons.

Any suggestions would really be appreciated. Sorry for my ignorance on 
dns issues.
  
btw: How/why does the hostname and internal ip of the laptop sending the 
message also show up? (johnny-lt.johnnystork.ca)

Recent bounced message:

The original message was received at Sun, 25 Mar 2007 08:03:04 -0700
from johnny-lt.johnnystork.ca [192.168.1.10]

   ----- The following addresses had permanent fatal errors -----
<job-295169023 at craigslist.org>
    (reason: 554 <unknown[207.216.240.22]>: Client host rejected: rDNS/DNS validation failed. Please setup matching DNS and rDNS records: http://bind8nt.meiway.com/itsaDNSmess.cfm)

   ----- Transcript of session follows -----
... while talking to mxi1s.craigslist.org.:

>>> >>> DATA
>>>       
<<< 554 <unknown[207.216.240.22]>: Client host rejected: rDNS/DNS validation failed. Please setup matching DNS and rDNS records: http://bind8nt.meiway.com/itsaDNSmess.cfm
554 5.0.0 Service unavailable
<<< 554 Error: no valid recipients


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Johnny Stork
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