Bitrix24 commercial plans subscribers can configure Bitrix24 branding in emails sent from Bitrix24.
To remove or add Powered by Bitrix24 signature from\to your CRM, go to CRM > Settings > Other > Other Settings:
Go to Activity section and scroll down to Outgoing Email Parameters. Here you can enable or disable Sent by Bitrix24 signature.
Please note that Bitrix24 Free plan subscribers have Powered by Bitrix24 signature enabled by default. In order to remove it you need to upgrade.