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The main cause of this error is wrong system date. Make sure that the system date on your computer date is set correctly and you should be good to go.
Hope this helps.
In the Mac OS the date and time settings should be in the System Preferences. If you cannot find the settings let me know and I will provide you with detailed instructions.
Is this the page you are trying to access? (note: if you click on the Check Email icon it will take you to the webpail sign in page)
If your system date is correct, then the expired certificates message is most likely caused by a problem with the site you are trying to access. You can confirm this by signing in to other secure sites and see if you get a similar message.
You can also try using another browser to access the site, clear your browsers cache and cookies, lower its security settings, and reboot your computer. If none of these resolves the problem you my need to contact the site itself.
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