I don't know what platform you are using for your device @Zephyr1 or what app you are using to access your email. Typically you would configure a signature for each email address on your device and rely on your device to manage signatures. If you are using the native app or access and it does not send perhaps you are using the mobile version and not the full fledged website version? On my device (Android) I tend to use Gmail to fetch and send my email and configure the signature per email address. Does any of that help?
...turns out that my two cents is worth less or more depending on the current exchange rate.
roy darling *my posts seem a lot shorter in my head