I know this is a year ago and it may have been solved in an update, but I've not seen a solution to it online, so here is the solution that helped resolve it on our end.
1. Go to the person's computer and open Outlook.
2. Go into Options
3. Go to "Mail"
4. Go to "Editor Options"
5. Go to "AutoCorrect Options..."
6. In the "Autoformatting as you type tab" Check off the "Straight quotes". Test it. If it doesn't work, go to the "Autoformat" tab and check off the straight quotes option there too.
Here's a picture of the options
I speculate the cause of this is a feature that is available in a lot of Microsoft office Apps and how apostrophes and single/straight quotes are used interchangably in a lot of interfaces. When a single quote is typed in within a composing message in Outlook, Outlook will try to convert this single quote/aprostrophe into a directional single-quote. Somewhere along the pipeline before the email heads out, the transcription service couldn't handle the directional single-quote character.
For an example of what they both look like, this is a simple straight quote, it's ascii code is 39:
'
And the below is a directional opening-quote that outlook replaces straight quotes or "apostrophes" when the user types it in:
‘
The character above was what is causing the &bsol to show up for our users. It's 1 pixel standing 1 space to the right. So much fear and doubt over so small a thing.
You can confirm this yourself on the problematic Outlook application. Copy and pasting doesn't seem to trigger Outlook's autocorrect of straight-quotes into directional quotes. So if you manually type in a single quote into a composing message in Outlook, and then paste a single quote from notepad into the same message you should be able to see the difference. If you send it, you'll see only one of them transforms into &bsol when the message is set to HTML. I'm not sure why this only happens to some computers, as it does not replicate reliably on others.
Why &bsol? I speculate this may have to do with some system trying to interpret Outlook's directional single quotes with some sort of escape character(\), but it gets stuck with the backslash for it, then fails silently. The backslash in ascii code is 92, the backslash in HTML code is &bsol (no semicolon though).
I have tried to puzzle out what \u9785 is in the original post, but without the original to compare I came up with nothing.
I don't have any concrete information about why this is localized to the users in one of our departments, but I think it may have something to do with copy and pasting text from a really old word file type into the draft of an Outlook email. The inclusion of an emoji probably triggers something similar.