Postfix needs to be updated to 2.1.4 from the 2.1.1 currently in the development tree. Earlier 2.1.x releases incorrectly cache state when falling through to secondary from primary. This can result in a situation where you cannot send mail when connecting to a primary MX, being blocked temporarily (4xx error) and failing back to a secondary MX. 2.1.4 fixes this bug....
Fixed in rawhide in rpm postfix-2.1.4-1 or newer.