Web Mail Frequently Asked Questions   
Can I change the Default view from the Calendar to my Inbox?
Sure,

Log in to your webmail account and click on the "Options" icon at the top of the screen
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Next, Click on Display Options
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Next, select the drop down for "What option should Horde display after login?" and select "Mail"
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Save your changes
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The next time you log in you should be taken directly to your Inbox.