Hi @mstudioil,
The red error you mentioned sounds like an error coming from CF7, not from WP Mail SMTP. However, if you can let us know what the specific error message is that you’re seeing, we can let you know if it’s something you should bring up to the CF7 support team, or if it’s something we can assist with.
Regarding the plugin removing data, to make WP Mail SMTP remove its data when you uninstall the plugin, enable the “Remove ALL WP Mail SMTP data upon plugin deletion.” option under WP Mail SMTP > Settings > Misc.
Here is a screenshot in case it will help:
https://d.pr/UZyLhd
I hope that helps! Have a great weekend!