(I did post a ticket to SM, but as they are closed now until Monday, I thought I'd post here to see if anyone else has the same and collect more data)

Since I upgraded to 7803, sending messages using MAPI is (sometimes) corrupting the special characters in them. I know it's not the inbox receiving part, because you can see the corrupted version of the message in the sent items folder.

For example:

Becomes:

Thinking about it, I guess it's possible it could be an issue when sending with non MAPI as well, but I have only noticed since I did the MAPI conversion for a large client.

Outlook 2016 and 2019 both give same results.