Apple's iCloud service is a convenient set of online tools for Lion Macs; however, if part of the service goes out then those who rely on it can be seriously burdened.
Earlier today iCloud's e-mail service was affected by a brief outage, which resulted in a number of users unable to check their e-mail accounts either via Mail or through the iCloud.com Web page. The service has since been restored, but this problem may have had a number of people needlessly troubleshooting their Mail application setups.
Luckily this outage was a brief one that lasted only a few hours for those affected; however, it may help to know where to look in the event you cannot check Mail, especially if you are about to modify settings to troubleshoot your Mail setup.
Apple has a system status support page for iCloud where it posts information about how the iCloud service is running, and also lists recent outages in the system. Today's outage is listed as having been out between 10:58 a.m. and 12:50 p.m. PST, so if you were unable to access your e-mail during that time, then this was likely the reason.
Apple's status page is available here, but can also be accessed by going to the "Support" section of Apple's Web site, clicking "Other Products," and then selecting the iCloud section of the support page. The current service status should be listed on the main page, but you can click the link underneath it to get to the detailed status page.
Unfortunately Apple does not supply too detailed of an explanation about the outages, but this should be a useful location you can check if some of your iCloud services are not behaving as you would expect.
The use of status pages is often overlooked when troubleshooting online services, even though numerous companies have them. Check the support section of the online services you use and make a note of where (if any) the service status indicators are. It may be useful to create a bookmarks folder of these service pages so they are easily accessible, but if they are showing any outages, then hold off on troubleshooting your software setups until they are cleared.