problem is fixed. i believe what happened is that i successfully added users prior to successfully having a valid email account. we had some issues we had to troubleshoot on the exchange side when i first set this up. I modified user's email from a valid one to an invalid one and then back to the valid one and the welcome email/validation email was re-sent. deleting and re-adding the user also works.
i also tried the following in order to have the user validation email re-sent and it did NOT work
enable/disable the user
modify the email to an invalid address and then back to valid without saving the changes in between