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1. Launch Opera and open Menu >> Mail and Chat Accounts.
2. Select your mail account and click Edit.
3. In the new window, open the Outgoing tab, then click Edit signature.
4. Select now whether to use HTML Formatting or Plain Text, then insert your custom signature. If you want to apply the new signature on all other mail accounts you have, check on Apply current signature for all accounts. When you finish, click Save.