What Criteria Must a Transaction Meet to Be Accepted into the Mempool?

To be accepted into a node's mempool, a transaction must first meet several validity criteria. It must be syntactically correct, not already confirmed in the blockchain, and all its inputs must be unspent and valid.

Furthermore, the transaction must adhere to the node's local policy rules, which typically include paying a minimum relay fee and not exceeding a certain size limit. If a transaction fails any of these checks, the node will reject it and will not broadcast it to other peers.