You can create an email signature via Webmail. All you need to do is log in to your Webmail interface, click on the icon gear on the top right and choose Preferences:
Click on Signatures and then on Add a signature. Assign a name to the signature and add the content. Once it's good to go, click Save:
That's it! From now on, the newly created signature will be automatically added to all of your messages.
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