I have this same setup.  This is the problem and how I solved it:
 
Your DNS entries are setup (presumably) to point to the public address of the SW, then are port forwarded to the IMail server.  However, the SW does not allow access to the WAN address from the LAN side.  Your clients are probably resolving the DNS address of the mail server as the WAN address and attempting to connect to the WAN port (which is not allowed).  Your clients on the LAN must use the LAN address to communicate with the IMail server. 
 
In my setup, I don't host my own DNS.  At least not for public consumption.  I setup MS DNS on a server and created a zone that contains all internal servers.  And since I'm using Active Directory, the clients use DDNS to add themselves automatically.  This DNS only resolves internal addresses.  Then I turned on the setting in that DNS to "enable forwarders".  When this option is set, my DNS server will ask the specified external DNS servers to resolve anything that it can't.  Then I point all of my clients to my own DNS.  (I also point all clients to the public DNS as a secondary, just for backup).
 
NOTE: Do not allow outside access to the internal DNS server!!!
 
Now internal clients will resolve the IMail server address, by name, as internal.  And the external clients/servers will resolve the IMail server address, by the same name, as external.  And everyone is happy.  In fact, my laptop finds the IMail server on my network and when I'm on other networks, without changing the POP server name settings.
 
If you want more info on this setup, contact me off list.
 
Todd
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Administrator2
Sent: Friday, April 12, 2002 3:03 PM
To: [EMAIL PROTECTED]
Subject: [SonicWALL]- iMail fails to connect through SonicWall from LAN

I have set up my iMail Server on a one to one NAT  (192.168.155.98) behinf my SonicWall DMZ.  I can send and receive mail from  machines  that have a public address but my mail now fails on our LAN.  Has anyone ever had this issue. I can access secure web mail from a pub
 
Ports that are open are 25, 110 and 443 (or SSL Web Mail)
Rules for each service:
 
 Allow  SMTP  *    192.168.155.98  Enabled
 Allow  POP3  *    192.168.155.98  Enabled
 Allow  SSL    *     192.168.155.98  Enabled
 
Anything missing?
 

Kevin M. Donovan, MCSE
Network Support Specialist
IntelliWare Systems, Inc.
 
Phone (817) 277-0800 x228
 
 

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