mail from: <> 571 sorry, sender address has invalid format (#5.7.1 - chkuser)
this is fixed in a newer version I hope?
Is CHKUSER_ENABLE_NULL_SENDER_WITH_TCPREMOTEHOST defined in your config ?
From the manual
CHKUSER_ENABLE_NULL_SENDER_WITH_TCPREMOTEHOST 2.0.5 defined
Enables accepting null sender "<>" from hosts which have a name associated to their IP