Hello @novy85,
Thanks for giving us more information related to the issue.
Since the spam email is not listed on the reports page, there are fewer chances that the email got sent through the plugin but we need to confirm it further through the below steps
1. Confirming if the issue is occurring consistently
Can you please try sending some test newsletters to some of your test emails and check if the spam email is also getting sent every time?
2. Checking for a possible plugin or theme conflict
Will it be possible for you to perform a conflict test on your site? You may need to temporarily disable all other plugins except the Email Subscribers plugin and switch to a default WordPress theme.
We recommend doing the troubleshooting using the Health Check plugin which disables all the plugins and switches the current theme to the default theme only for the current logged-in user when troubleshooting mode is enabled. It does not affect other users or site visitors. You can get the Health Check plugin from here. After enabling the troubleshooting mode, activate the Email Subscribers plugin, and check if the issue is resolved or not. If the issue is resolved, then it indicates a possible conflict with one of your active plugins or theme. Activate them one by one and check for the issue till you find the conflicting plugin or theme.
3. If the issue is not resolved then please allow us to further debug the issue on your site. You can open a support ticket here. We will then look into the issue from there.
Note: Please mention this thread URL while opening the ticket so that we can link back it in our private support channel.
Hope this helps you. Let us know what comes out or if you have any further queries.
Thank you.