554 Unallowed chars encountered

My Exchange v5.5 IMS server received an inbound internet-based message that it could not processed. The message header appeared to be corrupt and had a line that read, "Diagnostic-Code: smtp;554 Unallowed chars encountered." The message header also mentions UTF-7. All internet-based messages are routed through an HP SendMail server. Could this be caused by an character set incompatability between the sendmail server and Exchange IMS server? Ideas/suggestions appreciated.

This needs to be addressed either where the mail originates or where it is received. I do have a pc on my desk and it does pc mail. I sent myself an email from a HP-UX box and it came in as an attachment. I did not see utf-7 but rather roman-8. I was able to click on the attachment and read it. That test was done using mailx on hp-ux.

I tried again using the old "mail" program and this time I could read the message normally. Finally I gave myself a .mailrc file with a single line "set charset=iso-8859-1" and retried mailx. Again, it came in normally.

Microsoft does screwy things to say the least, but I doubt that this is an Exchange issue. On my pc I have a program called Outlook that actually lets me read the mail. My guess is that Outlook is the program that can't handle high-powered characters sets. I would kinda expect Exchange to simply move the mail around. But with Microsoft you never know