In the event of Outlook not responding, a few questions will need to be asked.
1. How long has it been unresponsive?
2. Is this the first instance, or where there other cases?
3. Is the computer resources being shared by other hungry memory apps?
4. What is the remaining size left available in the mailbox?
5. How many shared mailboxes does the user have?
6. What is the size of the user's OST file at 'C:\Users\xxx\AppData\Local\Microsoft\Outlook'?
If memory or CPU usage is high and a search or email being sent was issued during this time, the response to perform the desired command will be queued. This will have a long wait time, but should the wait time exceed 10 minutes, Outlook may require an End Task in Task Manager to kill the process.
If Outlook can still be interacted with but certain options do not allow a message to be previewed or a new message to be written, restarting Outlook may resolve the issue.
If a user's OST file exceeds 10 GB in size, performance will begin to suffer. A max of 50 GB will make Outlook unresponsive or inoperable, at which point will need to be deleted and redownloaded.