HTML emails and line length

I am trying to solve a problem with sending HTML messages
using Sendmail at at my paid for hosting website.

I am not certain about the cause but your input would be
appreciated.

It appears that when I send a multipart/alternative message
using MIME
there is a problem with the message after it has been
transmitted and is subsequently displayed on MUA screens.

The plain text version displays correctly and also the first
part of the HTML message BUT when you get into the
section of the HTML the decoded lines are so long they reach
the end of the display window (i.e. I think over 1024 bytes)
and the software is inserting an ! mark at the wrap position
which causes the message to display incorrectly in MUAs such
as Outlook Express and also web based MUAs. This is
especially true if the ! mark occurs in a link - the whole
link may not work

I am not sure if it is Sendmail, the mailserver, IE or my HTML plug-in (XStandard free d/l) or some
other gateway causing these problems.
I have seen long limes sent through other gateways but I
have never seen the ! mark inserted too.

I wonder if you can configure the server to stop
inserting the ! mark or wrap the lines at less than 1024
bytes
to stop the insertion of the ! mark.

Your thoughts on this would be much appreciated.
Antares