problems with php mail()

This morning I’ve installed the plugin Newsletter, and it worked perfect.
Until I deactivated another plugin WP-Mail-SMTP. Because I use the PHP mail(), and not the SMTP.
Because this disables de fuctionality of Newsletter, I make WP-Mail-SMTP active again.

But now nothing with mail works anymore. My contactform, sending password to email and Newletter.
It’s probably just one stupid thing, but I can’t find what.