Is this just to notify our balance? Its notify via email?
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Yes
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And what if received or sent btc. Will it have notification too?
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Yes
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But i see its like pointless actually.. most qallet have their own notification.
Most of online web wallets probably has notification functionality (?)
Personally I use several desktop wallets that don't have an ability to send notifications via email.
This is also useful if you want to watch address which you do not control (don't have private key)