I'm having (or at least had) the same *exact* problem. I thought it was something with my DNS provider (EasyDNS dot com), but everything seems correct...
#>nslookup -query=mx kinsleyrefrigeration.com
kinsleyrefrigeration.com MX preference = 4, mail exchanger = mail.kinsley
refrigeration.com
kinsleyrefrigeration.com nameserver = ns1.easydns.com
kinsleyrefrigeration.com nameserver = ns2.easydns.com
kinsleyrefrigeration.com nameserver = remote1.easydns.com
kinsleyrefrigeration.com nameserver = remote2.easydns.com
mail.kinsleyrefrigeration.com internet address = 63.169.84.58
ns1.easydns.com internet address = 216.220.40.243
ns2.easydns.com internet address = 216.220.40.244
remote1.easydns.com internet address = 64.39.29.212
remote2.easydns.com internet address = 212.100.224.80
I had this problem with each 4.0 system I upgraded to 4.1.1. My assumption remains that it's something with my DNS provider, as I haven't seen anyone else here post this problem aside from yourself. If it's a bug, I have no idea where to look as everything *seems* to be ok.
Please let me know if you find a solution, and I'll do likewise.
Does your email bounce? Mine doesn't. I've sent email to the above e-smith from different domains, and nothing comes back. It just goes into the ether. Does yours do that too?
I thought perhaps my ISP was filtering me, but going back to 4.01 fixes the problem for me.