Did you try to send using Joomla massmail if you get the same issue on smtp?
What did you do before this issue happens? if it was working before then somehting change in the configuration or in your server that causing the issue.
It is pretty standard, if it was working then it would be working until someone change it. It is like hitting your car on the wall when nobody maneuvers it.
May I know what version of jnews you are using?
Please let me know any updates.
Have a nice and pleasant weekend!