Mail errors.

I get this error when receiving a HTML email:

                            CORRUPTED MESSAGE

This is the Courier Mail Server 0.55 on server88-208-239-58.stmarkshospital.org.uk.

I received the following message for delivery to your address.  This message
contains several internal formatting errors.  This is often caused by
viruses that attempt to infect remote systems.  Instead of blocking
this message, I converted it to a safe, text-only attachment that can be
safely read with a text editor.

This sometimes also happens when the sender's mail software has a bug
that creates improperly-formatted messages.  Although these kinds of
formatting errors may often be ignored by other mail servers, this
server detects and intercepts improperly-coded messages in order to
prevent viruses from taking advantage of bugs in E-mail programs:

-----------------------------------------------------------------------------
This message contains invalid MIME boundary delimiter headers.

See 'IMPLEMENTORS NOTE' in section 5.1.1 of
for more information.
-----------------------------------------------------------------------------

Any ideas why?