This could also be due to a virus/malware lurking on one of your Client machines. I've seen something similar occur on a Customer's facebook account. All the laptops and tablets showed a weird language (I think it was a Pashtu Indian variant) when logging into their facebook account. Turns out it was a facebook virus and it propagated to all client machines that logged into her fb account. Fix was to use Norton tool that scrubbed the facebook account (facebook scan); it found the virus and removed it on her account. After all her machines were logged off of fb, the tool applied and viruses removed from the account, logging back in to fb removed the problem. Had to use the tool on all 3 of the computers she regularly used to login to her fb account however. 2 laptops, and 1 tablet. 1 of the laptops was her husband's; so it was unclear which of those 3 machines was the original infection vector. The lady, my Customer, blamed her husband!
Since you are running Office365, that's a web-based portal E-mail, so it will have to be treated like my facebook virus above in that you have to scrub each and every machine connecting to the E-mail web portal with a removal tool. You could write a script to do this. It could be tricky if you have hundreds of users logging into your Office365 (Outlook 365) client. If you are using this in a company environment, you may have a manual or digital list (in Excel or Access) of all your user's E-mail accounts. You'll have to make sure you get your script to run on 100% of your Users who ever login to your web portal so the tool can scrub
ALL the user E-mail accounts of the virus.
I've never seen your exact problem, but, I've talked to other E-mail administrators like me who've talked about similar issues here on the tech forums (also on Microsoft Community Forum). It really does sound like a virus to me.
You can also call Microsoft Support and get help from them, even if your Company doesn't have an annual support contract. MS can help you over the phone for
$79-$199 typically charged per incident. They will tell you they have an hourly charge, but you should ask for the
"Per-Incident-Support" option. As it could take 20+ hours over multiple days or weeks to resolve this kind of a problem, but you only pay for the 1-time incident charge instead of racking up tons of hours at their hourly rate.
Let us know how you get on.
Best of luck,
<<<BIGBEARJEDI>>>
