Outlook 2003 Sending Error Code (0x80070057)

Are your outbound and inbound mail servers the same for the pop3 account, or are there two different servers?  

the settings for the POP3 e-mail account are - Outgoing Mail Server is Set up as pop3.xxxx.net - port 110,  Our Incomming Mail Server is set up as smtp.xxxx.net port 587.  The xxxx is our company name.   These settings seem to work for everyone and have also worked from time to time on the machines that we have received the error message on.  In the Tools-Options-Mail Setup - Send Receive Groups - both accounts (Exchange Server and the POP3 account are set up to send and receive mail items).

Are the PCs having problems using Outlook 2003 as the email client?  If so, the problem could be with them - Outlook 2003 has some known problems with POP email that are generally corrected by applying Office 2003 Service Pack 1.

http://www.microsoft.com/downloads/details.aspx?FamilyId=9C51D3A6-7CB1-4F61-837E-5F938254FC47&displaylang=en

We have resolved the problem at our company.  We were running SBS 2000 with the built in Exchange Server package.  We suspected that our AD was corrupt (Had many unexplained problems), replaced the old server and took it off line.  We created all new user accounts on the new server but before we shut the old server down we exported everyone's files to a backup and restored them to in Outlook 2003 and the new exchange server - thus saving everyone's old e-mail and contacts book.  This is where we made our mistake.  Outlook remembers that last path/method it successfully used to send e mail to individuals.  (I am told this is in the Exchange Server certification test manual) It is in a field that you cannot see when you click on the "All Fields" - Email fields in a contact - and cannot change - you can however see the funky e-mail address that causes the error when you left click on the "TO" box to bring up your contacts list to select the recipients you want to send the message to - after you have selected a recipient, you scroll to the right of the name to view the e-mail address.  It might have had X400 in the address - I cannot remember but it was certainly not the standard or correct e-mail address and it was only for our internal exchange server users.  We had to delete the email address in our contacts list for our Exchange server and re-create them.  This was easier for us because we were using a "Company Contacts List" in a Public folder so they only had to be changed in that book.  We also deleted any exchange server names in individuals "personal" Contacts so they had to use the public "Company Contacts List" and made that list the first one that appears when you click on "TO" when sending an e-mail.  We also had to make the POP3 account the default e-mail account.  We also had a problem with the fact that our POP3 account was for xxxx.net and so was set to the same address - this caused e-mail to bounce back an forth and not get delivered.  We changed our local exchange server to xxxx.local. I took care of the problem myself with some help from our computer consultant - I just never got the time to put my own solution on the experts exchange.