Yeah I know Outlook 2003 is out of support now, but for those who still use it…
Users were noticing that when they sent longer emails, large chunks of the emails were not sent. That is they were truncated at some seemingly random point in the text. It came to light when recipients were replying and asking where the rest of the email was!
The soulutions suggested were that it was Internet Explorer 11 that was causing this. I tried uninstalling IE11. It came back. I tried uninstalling IE11 and unticking the ‘Don’t automatically install updates’ tick box on the ‘Help/About Internet Explorer’ menu option in IE10 that was left on the machine. IE11 came back. I tried doing the same again AND installing IE11_BlockerToolkit which is designed to block the installation of IE11. It came back.
Ultimately the solution that has worked was to uninstall IE11, restart and check for updates, then hide the update. Bizarre but it worked. It took ages to come to this point, nothing seemed to work as it should until then.
Hope that helps someone. Oh and of course, when you fix it, use Chrome or Firefox anyway!