I have this problem with subscription of newsletter but the address is successfully stored in the Database.
I have magento 2.2.2 fresh installation on server and allows php to send emails.
system log says
main.CRITICAL: Exception message: Unable to send mail. Trace: #0 /var/www/html/projectName/vendor/magento/module-newsletter/Controller/Subscriber/NewAction.php(136): Magento\Newsletter\Model\Subscriber->subscribe('test@domain...')
Can someone help me on fix this error.