It is probably a problem with bounced email encoding. There is Check bounced emails scheduled task, which runs once per hour by default. The class responsible for it is CMS.Newsletters.BounceChecker, in there we have an a event OnFallbackDecoderEvent - which occurs when a character set from email content type header could not be mapped to the encoding. This allows to map given character set name to the encoding manually. I would raise this issue with support and ask them if it is possible to attach your custom handler to this event.