POP3 and Exchange Account - calendar invites fail via pop3

we have an sbs2003 server running exchange. users have outlook 2007. the default account is pop3 (going to pst file) and the exchange account is used only for shared public folders and archive. when a user creates a calendar event (local calendar from pst files NOT shared public folders) and invites others and than sends the e-mail the message is immediately returned "system undeliverable" - with the following error

550 5.7.0 From address mismatch: envelope <NAME@WEBADDRESS2@.com> and header <NAME@WEBADDRESS1.org>

i have switched out the real name with NAME and the real domain names WEBADDRESS1 & WEBADDRESS2. they are 2 different e-mail addresses entirely and the exchange server is not active to the outside world what so ever. it does no sending and no receiving just sharing and archival purposes right now. any thoughts why this happens. even if i select the the default account as the account to send from.
jsgouldAsked:
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peakpeakCommented:
Why are you using POP? I tried an invitation through IMAP and it worked.
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jsgouldAuthor Commented:
the exchange server is not configured to to send or recieve e-mail as per the original question. and before we added the exchange server it worked fine.
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