Could not complete the operation, one or more parameter values are invalid

Have you seen this? It's for Outlook 2002 but the troubleshooting steps should work on 2003 as well... not sure which Outlook version you have.

Ooops... here's the link http://support.microsoft.com/kb/320842

This is not a pop3 account and it only happens when this user send stuff to the president of the company with attachments, it does not happen any other time... strange!

I'm not sure what POP3 has to do with anything, it's not mentioned in the article... but did you try recreating the profile as the article suggested?

If the new profile has no effect, try this article http://www.msoutlook.info/question/19 to reset/rename your DAT files. This article will work for Outlook 2003 and 2007 and Windows XP, Vista and 7.

It would help the troubleshooting process if you would post your Outlook and Windows versions.

sorry, it's Outlook 2010, and Windows 7

these articles are about toolbar and tasks, he is having trouble sending emails, and not always, he looked on the people who he sends to when he gets the erros, and one person is always there. I don't know if it makes any sense!

The reason I mentioned pop3 was that when I read that article it said something about pst file, and here I do not have a pst file.

Recreating the profile and renaming the .DAT files are standard troubleshooting procedures for Outlook. Please try them so we can eliminate the possibility that those simple fixes will work.

Actually don't bother with renaming the DAT files, 2010 doesn't use them.