Unprocessed characters in emails

:D Hi! I'm just wondering why some emails received by our SMTP server contain unprocessed characters such as =0D=0A. Sometimes the file attach to the email contains an invalid path like this:

__Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename="=?ISO-8859-1?Q?20022850MANAGSUON=A0BROTHERS=A0PACKAGING=A0CORP=2E=2EXLS?="_

It would have been easy to replace these characters with their equivalent, but more and more different characters appear in different emails everyday. Could it be a problem with our SMTP receiver? But Microsoft outlook express doesn't have trouble displaying the right content of the email. Can anyone make an explanation on this?