Honestly, as we’ve written here (Please read this, before you start!) and always try to communicate: Hestia isnt a replacement for sysadmin knowledge, we expect that you can basic maintain (read and understand logs, know the basic exim commands or know to google "exim cheatsheet and so on) an exim server on your own - without the control panel. You got stated multiple times here from non-team (!) members, to get “professional” help from a specialist which have the missing knowledge.
In your current case, and again, due to less informations wild guessing, I would say you need to clean your mail queue.
Also, please don’t try to blame hestia for a issue that’s clearly on your side:
It isnt the control panel, also I dont think that the default exim template - which you probaly have overwritten during an upgrade (?) - includes an open relay configuration. So your basic issue is still unknown and I would strongly suggest to get someone to help you out here…