In my case, I can't send any email the default email system also no working. For checking, I sent the order confirmation mail it is not sending. it shows the error alert "Unable to send mail: Unknown error"
I had enabled all features but it is not working.
Check Stores->Configuration-> Sales->Sales Emails->Order . Enabled is "Yes".
Check Stores->Configuration-> Advanced->System->Mail Sending Settings. Email Communication is enabled.