Error 0x80070057 when send meeting requests

[Israel]

I have a user who has sent a meeting request out to a
number of attendees, however he is getting the following error message:

This message could not be sent. Try sending the message again
later, or contact your network administrator. Error is
[0x80070057-00000000-00000000]

Everytime he tries, he receives the same error.
Some of the attendees are within the company, other are external accounts,
he has never had problems sending messages to them in the past.
He was using Office 2003 when there was any problem, now he has Outlook 2007
and receives the error.
He is typing the address, and selecting from GAL the attendees.

Thanks in advance for your help.

Israel Gonzalez.

 

[Evolution]

I also just had a user with this exact same issue. The curious thing is that
someone else within the organization sent out the same meeting request and it
went through. All of the email address settings are correct.

Thanks for the help with this, I am at a loss now for trying to figure out
what is causing this.