Using Classic Outlook on Windows for personal email, calendar, and contact management
This sounds about right. I've had this issue before with a client, they were using Thunderbird, receive an email from someone on an iPhone and the Chinese characters would show up. Had to adjust the Unicode and it went away.
I'm suspecting it's the same with this issue. People are replying from another email client or the Unicode on the sender/receiver is out of alignment.
Hope this helps.