Hello,

 

    I don't believe you can enforce a signature from an IMAP client. You can however establish a footer for your whole system and per domain, using the Footer settings found in the System Admin > General settings> Footer card. You can also find them per domain, if the system checked "Domains can override footer", inside Domain Settings > General > Footer card. This will let you enforce a footer at the bottom of all messages coming out of that domain/system.