7. Signing/Verifying a message
Bitcoin messages are useful because they can be used for proving the ownership of a certain address. There is no standard for signing messages for nested and native SegWit addresses. That is why it is recommended to use legacy type of addresses. To sign or verify a message go to "Tools -> Sign/verify message".

Type in the message you want to sign and the address that is associated with your private key. Otherwise, it won't work. Click "Sign".
You should always include a timestamp and the purpose of the message to avoid reusing it by a scammer.

You should use the format below if you want to send the message you have just signed to someone else.
-----BEGIN BITCOIN SIGNED MESSAGE-----
<insert message here>
-----BEGIN SIGNATURE-----
<insert address here>
<insert signature here>
-----END BITCOIN SIGNED MESSAGE-----

To verify a message, paste all the required information and click "Verify".
If you want to learn more about Bitcoin messages then this tutorial will give you plenty of information. You should consider staking your address here if you are an active user on bitcointalk.
8. Extra information
Electrum is one of the most popular Bitcoin software wallets. It is constantly being updated so things described above can be a little bit different. At the time of writing this tutorial, the latest version is 3.1.3. I will do my best to keep this tutorial up-to-date and add more information in the future. I didn’t have enough free time to test two-factor authentication wallet so I will do it soon.
Electrum 3.1 finally brought us mempool based fee estimation. You can enable it in “Preferences” and use it instead of ETA which might be sometimes not accurate and cause inexperienced users to overpay for their transactions.