Couple of common troubleshooting tips:

1. Start up in Safe Mode

Follow these steps to start up into Safe Mode.

-Be sure your Mac is shut down.
-Press the power button.
-Immediately after you hear the startup sound, press and hold the Shift key. The Shift key should be pressed as soon as possible after startup, but not before the startup sound.
-Release the Shift key when you see the Apple logo appear on the screen.

After the Apple logo appears, it may take longer than usual to reach the login screen. This is because your computer is performing a directory check as part of Safe Mode.

2. While in Safe Mode, repair disk permissions using these instructions:

-While viewing your iMac desktop files and folders, use the Finder menu at the top to navigate to the Utilities folder by clicking on Go and then Utilities from the drop-down menu.

-In the Utilities folder, scroll the list until you see the application called Disk Utility. Double-click or single-click the Disk Utility icon to open the application.
-In the left sidebar window, highlight the name of your hard drive (Note: It's the item listed directly below the top entry.
-In Disk Utility, three tabs should appear near the top right side of the screen. Disk Utility should default to the First Aid tab and will be highlighted in blue. If it's not selected, click it once.
-Several button choices should appear near the bottom of the Disk Utility window. Select Repair Disk Permissions and wait for the process to complete (Verify Permissions and Verify Disk only go through the motions and don't fix anything).

3. To leave Safe Mode, restart your computer without pressing any keys during startup.

4. If you're running OS X 10.7 or later on your iMac, you should have access to what's referred to as a Recovery System Disk that would allow you to run the Repair Disk feature to fix issues associated with your startup drive. Accessing the Recovery System Disk is not difficult, but I will reserve giving you the exact instructions until after you run Repair Permissions while booted in Safe Mode.

Instructions for troubleshooting your email program depends on what software (Apple Mail, Entourage, Outlook, or some type of webmail app such as Gmail or Yahoo) you're using.