How the emails is handle? Can I use my own SMTP like mailgun or provate smtp?

I am planning to have and average of 1000 forms filled by day that will require 2 emails

Yes, FormNX does provide an inbuilt default SMTP that you can use OR you also get the option to enter your own SMTP from Gmail/Mailgun/Others.

FormNX - A No-Code Drag-n-Drop Form Builder.