We are facing issue with Magento 2.1.15 regarding emails customer not getting any emails from our store:
- after placing order.
- after invoice
- after shipment
- forgot password
- new account register
only contact form is works fine.
i setup all necessary settings but still it is not sending mails.
- Disable Email Communications is set to No.
- Asynchronous sending is Disable
- Under Order tab Enabled is Yes
- Mail sending Settings correctly
table cron_schedule has some job code called: sales_send_order_email ssuccess
var/log/system.log: main.ERROR: Unable to send mail.  
Please help me to solve this issue
Please have a look on this blog, this will help you debug your issue
still not solved