MX Lookup Tool – Check Your DNS MX Records Online
http://www.mxtoolbox.com/

This test will list MX records for a domain in priority order. The MX lookup is done directly against the domain’s authoritative name server, so changes to MX Records should show up instantly. You can click Diagnostics, which will connect to the mail server, verify reverse DNS records, perform a simple Open Relay check and measure response time performance. You may also check each MX record (IP Address) against 147DNS based blacklists. (Commonly called RBLs, DNSBLs). This has been added to the tools section of Research Resources Subject Tracer™ Information Blog. This has been added to World Wide Web Reference Subject Tracer™ Information Blog.