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.