Adding Mailboxes in Outlook 2002

[Lisas]

I've added two peoples mailboxes to another person's Outlook and one of the user's mailboxes can forward messages without any problem while the other user's mailbox when tyring to forward an email comes up with this message"You do not have permission to send a message on behalf of the specified user".

I've gone into the properties of both the mailboxes that I've added then looked at the permissions and they matched. Is there something I'm missing or need to look for?

Thanks,

Lisa