Unicode error

[Robert Fitzpatrick]

We have an issue with a stock Postfix 2.11.3 sending to one MTA. The 
message is being sent from a PHP application to the localhost Postfix 
for delivery. After making header adjustments to satisfy envelope sender 
rejections, the MTA is now accepting with an NDR. Their IT sends this 
error from their logs.....

Input data are not Unicode-conformant or conversion between Unicode 
formats failed

I don't find much info on this error and have never encountered, can 
someone here help us to understand the issue?

-- Robert