The Electrum attack works because people trust their wallet history and only glance at the end of an address. The attacker's address matched the last four characters of the real one.

• Update Electrum to 4.8.3 or later on every machine that holds a wallet.
• If a transaction you just sent shows up with a different address, an odd near-zero fee or as "Local", don't bump it. Check the txid on a block explorer you trust.
• On a hardware wallet, compare the whole address on the device screen for every signature, fee bumps included.
• For larger amounts, connect Electrum only to your own Electrum server or node.