3CX DLL-Sideloading attack: What you need to know
For some time now, some emails that I have been sending from Outlook are bouncing.
At the moment an email arrives informing me of the following error:
554 5.7.1 Service unavailable; Sender address [firstname.lastname@example.org] blocked using uri.cal1.sophosxl.com
Does anyone know how it can be solved?