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So if you decided to use your Electrum and it's not an old legacy wallet with '1' addresses, you should include a note stating that it's signed with Electrum.
Otherwise, the recipient might use an incompatible client or message verification tool and fail to verify your message.
What is the problem with legacy addresses starting with 1 and why can't Electrum sign them, and how can it be solved?
You misunderstood, the problem is other clients' or tools' compatibility with New Electrum's signed message, Electrum can sign message with non-legacy addresses.
But the note I mentioned is recommended since other clients do not follow the same message signing/verification scheme as the one implemented by Electrum.
E.g.: If the recipient of the message used Bitcoin Core to verify, it will not be verified. (
the error will be specific to the mentioned issue)
That's where the note is for, they'll use Electrum or try other clients instead once they read it.