Email nightmare

SInce being forced to migrate my hosted clients over to 1-Grids Plex hosting the nightmares started and are still not resolved today 7/12/2020. Clients hosted on 1-grid cannot send email to certain other companies also hosted on 1-grid. They get a " no such user exists " error. When i change the smtp setting to something else ( not 1-Grid ) then the email goes thru without a hitch. Now logic tells me that there is a problem on the routing side of 1-grids email when sending that is wht it works with a different smtp server....but no one can find a solution......its been given thru to the " engineers" .....still no answer......useless...really. Even Zuber who sorted out other issues before is missing in action....