@Ben-Stokes oauth2-multiple is just newer and doesn't require the dev to fork the plugin.
You just install it and configure the information inside the ACP settings page.
Give that a shot instead?
What version of NodeBB?
You should see the relevant error in the output log. You should be able to capture the message by running ./nodebb log and then trying the test email.
Have you tried the same SMTP settings in a separate email client?
B
