If you are using any Avast Antivirus, including the free version, Avast automatically inserts an e-mail footer signature into all your outgoing messages.
The signature will looks like:

‘This email has been sent from a virus-free computer protected by Avast‘ if you are using Avast 2016

If you don’t want this message, just follow this steps to disable/remove it from your e-mails:
1- Go to your Avast interface by double-clicking on the orange icon in the Windows system tray. Or you can find Avast in the Windows Start menu » All Programs » Avast.
2- Go to ‘Settings‘ by clicking on the cogwheel icon in the top-right corner of Avast user interface.
3- In the ‘General‘ section find option ‘Enable Avast email signature‘ and simply un-check it.

That’s all – Avast will not include an e-mail footer signature in your outgoing e-mails anymore.