VOX emails a Disaster!

I have been struggling for more than a week with emails not being received by recipients, whilst showing as sent, and with first indications more than a week later that it has not been sent. Some emails goes through and some not - but on my side all show as SENT. It leaves me with the dilemma that I have absolutely no idea if my emails have really been sent or not. Some of the information and instructions relates to critically ill patients with life-threatening diseases. Speaking with the Vox email support line they say that the only way that they can assist me is if I also buy Office 365 from them. This forced up-sell due to their inability to fix problems is outrageous! l cannot change to a new provider as I cannot change my email address and I cannot port my email address to another capable provider as with a cell number. Vox say that they don't know how to fix the problem and it looks like major international providers are blocking email coming from a Vox/Absamail address as spam and won't let it through their firewalls. Surely you have the ability to fix this Vox!!!!!