I have appointment+ plugin on my site, earlier everything was working fine but after I made version update of Appointment+ plugin the confirmation emails stopped going out.
I've tried with SMPT and without it. With SMTP in WP Mail Logging plugin I got an error:
SMTP error: data not accepted
And no errors without SMTP, but still can't receive an email from Appointments
I also tried to disable all plugins, but this doesn't help. There is no debug.log