In order to find the root cause for this issue we need to look at a few different issues.
Is users inbox full? Is there any free space left in order to receive new messages?
For example; if users inbox is full at the email exchange server level then it may not be able to receive new messages (usually limited for each user). You can check this by looking at the folder size of user’s inbox. Of course you would need to know what the limit size is first.
Issue could also be caused by cached data at local level.
For example; you can have cached data in Outlook that needs to be flushed in order for emails to start working again (can be caused by file indexing).
What else can we do?
You can free up space by deleting old emails and calendar settings that may contain large attachments. Yes, calendar settings are stored at server level.
Sometimes, user would change their domain password while still logged on to computer but not able to receive emails afterwards…this is caused by Outlook not updating the user credential. Simple Outlook restart should fix this…