Some of you (ok, none of you) may remember that I posted back in 2004 about a cheap kludge to get around some Earthlink DNS bullshit. Well, it turns out that it may not be Earthlink's fault so much.

I'm not going to take the time to vet the blame here, but it seems that if you
configure Exchange to use your ISP's DNS servers (and not your internal Microsoft DNS server) the problem goes away.

Start Exchange System Manager, select Servers->(SERVER NAME)->Protocols->SMTP. Open the properties for the Virtual SMTP Server, select the Delivery tab and click on the 'Advanced...' button. Then clicking the 'Configure...' button and add the IP address(es) of your ISP's DNS server(s) here.

Special thanks to Ian Griffiths, as his blog tipped me off to this solution.

IanG on Tap: Exchange, Earthlink, and SMTP 550 Errors on SBS 2003