This usually happens when a 3rd party server is emailing you which includes characters which causes emails to be rejected. Microsoft used to remove these characters before, but they stopped doing so.
As per Microsoft documentation, receive connector need to be adjusted to fix this but that applies to on-prem exchange only. I couldn’t find anything for Exchange Online.
I fixed this issue by creating a transport rule on Exchange Online. Following is an example but you can scope it down to as per your needs but the main point is the condition, Append the disclaimer ‘ ‘ and fall back to action wrap if the disclaimer can’t be inserted.
You will have to click on More Options in order to get all these options under transport rule.