Re Internal Server Issue 500 at index.php?route=checkout/register/save

I am getting the following error while registering new user in checkout process: Failed to load resource: the server responded with a status of 500 (Internal Server Error) at index.php?route=checkout/register/save

These are the apache error Logs:

[Wed Sep 27 05:50:58.710132 2017] [:error] [pid 1016] [client x.y.z.a:58234] PHP Fatal error: Uncaught Exception: Error: AUTH LOGIN not accepted from server! in /var/www/html/ecom-intellobots/system/library/mail/smtp.php:173\nStack trace:\n#0 /var/www/html/ecom-intellobots/system/library/mail.php(81): Mail\\Smtp->send()\n#1 /var/www/html/ecom-intellobots/catalog/controller/mail/register.php(44): Mail->send()\n#2 /var/www/html/storage/modification/system/engine/action.php(51): ControllerMailRegister->index('account/custome...', Array, 24)\n#3 /var/www/html/ecom-intellobots/system/engine/event.php(37): Action->execute(Object(Registry), Array)\n#4 /var/www/html/storage/modification/system/engine/loader.php(192): Event->trigger('model/account/c...', Array)\n#5 /var/www/html/ecom-intellobots/system/engine/proxy.php(25): Loader->{closure}(Array, Array)\n#6 /var/www/html/ecom-intellobots/catalog/controller/checkout/register.php(207): Proxy->__call('addCustomer', Array)\n#7 /var/www/html/storage/modification/system/engine/action.php(51): ControllerCheckoutRegister->save()\n#8 /var/www/html/ecom-intellobots/catalog/contr in /var/www/html/ecom-intellobots/system/library/mail/smtp.php on line 173, referer: http://domain.com/ecom-intellobots/inde ... t/checkout


Thanks in advance.

This thread should help you on what this type of error message represents: viewtopic.php?f=170&t=165657#p630707 .