You can configure your clients' Gmail accounts in Thunderbird. When composing an email, pay special attention to the "From:" field. It is a drop down list that

You can configure your clients' Gmail accounts in Thunderbird. When composing an email, pay special attention to the "From:" field. It is a drop down list that shows all the email accounts configured in your Thunderbird. Choose the one you need to use to save your client's face. A more sophisticated approach is use the Identities feature of Thunderbird. https://support.mozilla.org/en-US/kb/using-identities https://support.mozilla.org/en-US/kb/using-identities