Session Reject (35=3)

Before your order reaches a book, the session may reject the frame itself.

35=3|45=12|371=35|372=D|373=99|58=Invalid MsgType|…|

Tag	Role
45	RefSeqNum — which inbound seq failed
371	RefTagID — offending tag if known
372	RefMsgType — offending message type
373	SessionRejectReason
58	Text — human hint

Session reject ≠ business reject. Fix encoding, length, or header before resending application flow.