Hello! Whether it will work or not will depend highly on how the signature software interacts with outbound messages. If this is something running in the user's email client that simply appends the signature on the way out - that should work well. I recommend testing this thoroughly before implementing it in production though just to be safe!

I wanted to follow up on this one more time with a point I forgot to mention originally. If you are using DKIM message signing with these domains your third-party signature system may cause DKIM validation failures on those sent messages, leading to a DMARC failure for the domain. 

This is because DKIM signing for any given message will only validate securely if the message has not been modified after being packaged and sent by SmarterMail. So, you may want to confirm at which stage the signature is being appended to avoid DKIM/DMARC failures. 

Additionally, we offer some built in functionality that may accomplish this without the need for a third-party signature system. Please see below for more information about this Domain Footer functionality: 

I hope that helps point you in the right direction. Have a good one!

We also have domain wide signature features when users are using Webmail that you can force or encourage by default etc.