SMTP Stops Internet from working

Windows 2003 R2 Svr, Exchange 2003 Svr., DSL connection all nestled nice & securly behing a SonicWall Pro2040 (or so I thought). Things were running fine for quite sometime, then, over the past few days, the Intenret started to slow down, & yesterday it quit working all toghter, no replys back when a ping was done to an outside name server. I have narrowed the problem down to the SMTP service. If I disable that in Exchange, the Internet comes back up & seems to be working fine. Responce times are a consistant 23, 58, 79, 23ms, as they should be. As soon as I enable the SMTP service my pings almost instantly show signs of a problem with responce times as follows; 384ms, 2800ms, 3900ms, time out, time out, time out.. and all bandwidth seems to be comsumed by the SMTP service & poof, no more Internet for any of my internal clients. Few more details to help in determining the problem.. Both Server & Exchange are completely up-to-date with MS updates. We are running Symantec Enterprise Ed. v.10.1 & it is up-to-date & functioning fine. This is not an open relay, but we do have POP3 running as we need to serve some remote people. The SonicWall Firewall seems to be as secure as it can be, & I actually have all incoming SMTP traffic tightened down to only let this type of traffice take place on a certian set of subnets. The reason for that is we use a third party Anti-SPAM/AV filtering service called MXLogic. All MXrecords point to MXLogic & they get hit with all the Crap mail & then the good filtered mail gets sent along its way to our Public IP from MXLogic. It almost seems as though I am getting hit by someone inside the network... Or could there be a bunk email stuck in a que somewhere? Any thoughts or suggestions would be greatly appreciated. Thanks.