Thanks @alanhericvoro
We were hoping to avoid this situation by re-namespacing Guzzle like
namespace SMTP2GOWPPlugin\GuzzleHttp;
I think generally this solution had stopped Guzzle conflicts, however in your custom setup it looks like it isn’t sorting this. We will look to update Guzzle in an upcoming release of the plugin: will probably be a couple of weeks away as we have an interim release planned for next week just updating WordPress version compatibility and our pre-release testing process
Really appreciate the feedback, thank you