As a technical support ,if the end user said that he cant send or receive e-mails , how can you fixed?

Check the internet connection,, and clean ur temp files and delete ur cookie and history file u could find a solution..... and if not restart ur pc. hope u ll find a quick solution.

Solution:    Verify that Outlook is online.

• On the Outlook menu, make sure that Work Offline is not checked.

Cause: Some POP and IMAP e-mail accounts use an outgoing mail (SMTP) server that requires authentication.

Solution:    Turn on SMTP authentication for the POP or IMAP account.

Some e-mail services, including MobileMe mail accounts, require authentication for their SMTP server. If you verify that all your account settings are correct, but you still can't send messages, try turning on SMTP authentication.

1. On the Tools menu, click Accounts.
2. In the left pane of the Accounts dialog box, select the account.
3. Under Outgoing server, click More Options.
4. On the Authentication pop-up menu, select the authentication type, and then enter your credentials as necessary.

Cause: Your ISP or network has a firewall that blocks connections to your e-mail service's outgoing mail (SMTP) server.

Solution:    See if your ISP or network administrator can provide an alternative SMTP server to use for your POPor IMAP account.

If you have the address of an alternative SMTP server, enter it in your account settings.

1. On the Tools menu, click Accounts.
2. In the left pane of the Accounts dialog box, select the account.
3. In the Outgoing server box, type the server name or address.

 NOTE   If you use a laptop computer in multiple locations, such as at home and at work, the SMTP server that you use in one location may not work in other locations. Each Internet connection that you use may have afirewall  with its own restrictions.

Cause: Your credentials or account settings are incorrect.

Solution:    Verify that your account user name, password, and other settings are correct in Account preferences.

1. On the Tools menu, click Accounts.
2. In the left pane of the Accounts dialog box, select the account.
3. Make sure that you have entered the correct e-mail address, user name, and password.

If your account still does not work properly, check with your e-mail service provider for details about how to configure your account in a mail application. Your account may require that you use special settings such as Secure Sockets Layer (SSL) to connect, or that you override the default port. 

Cause: Your network requires a proxy server to connect with the mail server.

Solution:    Specify a mail proxy server in your computer's System Preferences.

If you use a network to connect to the Internet, you might have to specify a mail proxy server. Ask your network administrator for the correct settings for your mail proxy server. For instructions about how to enter these settings, see Mac OS Help.

Cause: Your e-mail server is unavailable.

Solution:    Make sure that the mail server is online.

There are many scenarios in which this could happen. No internet connection, wrong email server settings, Password may have expired or changed, Mailbox reaching its full size limit, in case of outlook PST size limit is also a cause. But in lot of cases end use encounter an error message which will help a technician pin point the cause and troubleshoot the issue.

The main reason for this issue is his pst's allowed storage limit is full.In this case you will have to move or delete some of the mails from his pst.It will take some time to take in effect  even if you delete or move.so don't be confused.

I agree to Charbel Hanna and Safeer Andarath

Yes i agree with Safeer Andarath.

check the internet connection first or the incoming and outgoing server