We`ve reached a new milestone as of October 2020 – there are now over 1000 articles in the blog below – so scroll down or search for your issue. If you can`t find what you are looking for send me a question! Thanks to all my subscribers for viewing my website and watching my Tech, Watches and Travel videos on YouTube.
This is a classic relaying issue, where you have domain users split between the main mail server and this subsidary one. Untick the box that says “Do Not Try to Relay MailsAddressed to Non-Existent Users”