There is 2 type of bitcoin adresses prefixes .
Originally those starting with 1 is Pubkey hash (P2PKH) addresses . This with 3 is Script hash (P2SH address).
P2pool code originaly accept only P2PKH address for mining user . Similar to MUE P2PKH address start with 7 and is accepted by p2pool code as user. P2SH address isn't accepted on any p2pool mined coin. P2SH addresses are generated with web wallets .
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