Mail app updating cache directory
Microsoft Exchange Server 2010 analyzes the message, including the list of recipients to which it's addressed, and if it detects a potential problem, it notifies the user with Mail Tips prior to sending the message.With the help of the information provided by Mail Tips, senders can adjust the message they're composing to avoid undesirable situations or non-delivery reports (NDRs).
Mail Tips are informative messages displayed to users while they're composing a message.
When I looked again after a few days, the new users were missing again.
This same symptom is happening to my Outlook/Cached mode clients. Have seen some issues when migrating from Exchange 2003 to 2010 on users who use cached exchange mode.
The Mail Tip indicates the recipient has Automatic Replies turned on and also displays the first 250 characters of the automatic reply configured by the recipient. If the message isn't immediately sent, the Mail Tip is updated every two hours.
If part of your user mailboxes are hosted on Exchange Online and you're in a coexistence with Exchange Online scenario, the setting on the remote domain object that represents the remote part of your organization has a direct effect on how this Mail Tip is processed.