What Criteria Must a Transaction Meet to Be Accepted into a Node’s Mempool?

For a transaction to be accepted into a node's mempool, it must meet several validity criteria. It must be syntactically correct, not already confirmed in the blockchain, and all its inputs must be unspent (UTXOs).

Additionally, it must pay a minimum relay fee, not exceed the maximum allowed size, and adhere to the node's specific mempool policy rules, such as those governing RBF or ancestor/descendant limits.