Hello my father's small crane rental business email account for Office 365 has been hacked and sent out emails to everyone he has communicated with. The emails contain some kind of falsified OneDrive link and is somewhat personalized our business as it's essentially pretending to be a crane vendor agreement of some kind. I would say it could very well be a hacking of our gmail account as we currently have that configured to send emails as though they come from our Outlook address, however I was able to locate the hacker's sent emails in the Office 365 folders on our Outlook desktop app and was unable to locate any in the gmail folders.
-We were recently forcefully migrated from Godaddy email servers to Office 365 servers and have yet to migrate over any form of contact list, so I am somewhat confused as to whether someone has the capability to automatically generate a contact list out of all of our communications to send the emails to. As it stands, I'm not even sure how I should go about emailing everyone that received the compromised email.
-We have also been victim recently (within the past 5 months) to a form of email-bombing that was an attempt to obscure fraudulent credit card purchases and we are still having issues sorting out quite a bit of junkmail from worldwide, but mostly russian sources.
-I have seen certain emails over the past year that make a strong attempt at targeting to our business but contain enough vagueness to be discerned as someone's attempt to get us to click on a certain attachment. The emails contain enough information to definitively appear to be personalized as opposed to being related to some form of mass emailing scam.
So as it stands, I have changed all passwords to what I would consider incredibly tough to hack, am going to enable a second form of authentication using my father's cell phone when I can see him next, and am in the process of tracking down any possible security questions that may have been compromised. I would greatly appreciate any input from those well-schooled in this domain. I have read through certain articles that have suggestions on how to proceed, but am interested in hearing any more information that may be more pertinent to my situation.
Thank you for your time.