Conflict with Sparkpost

Hello. I use Sparkpost service and as SMTP because I have some 30K newsletter subscribers to send newsletters to 2-3 times per month. I also use WPForms for our “contact us” form. I thought I had to also install WPMail SMTP in order for the contact form to work. But as soon as I installed WPMail SMTP I got in admin: “Heads up! WP Mail SMTP has detected SparkPost is activated. Please deactivate SparkPost to prevent conflicts.”

1) Why is there a conflict?
2) Does WPMail SMTP perform same functions as Sparkpost?
2) Will WPMail SMTP be able to support the volume of newsletters and transactional emails from my server if I deactivate Sparkpost?

Thanks