Hi Brandell7,
Have you tried the steps in that link Jerry provided?
In addition, may I collect the following points to narrow down this issue?
1. Doe this issue occur when sending emails to a particular recipient or all the recipients?
2. When sending the email, does the user use the auto complete feature to fill in the recipient address or choose the recipient from the address book? Please try todelete the nick name cache in Outlook client, and then check if this issue persists.
About the limitation, how many recipients have you typed in the To box? How many messages has the user sent on this day?
Thanks,
Young Yang