Once you're in your folders, iOS is easy to use for most operations, such as deleting messages and moving them. You also can add and delete mailbox folders. iOS lets you easily search for mail by From field, To field, Subject field, or entire message. In the message list, you can delete, move, and flag multiple messages, though you can't select or deselect all messages.
When reading an individual mail message, you can delete, move, flag, reply, or forward it, as well as mark it unread or add the sender or a recipient to your address book. Unfortunately, iOS 5 has no way to view just your flagged messages, reducing this feature's utility. iOS also remembers the email addresses of senders you reply to, adding them to the database of contacts they look up automatically as you tap characters into the To and Cc fields.
iOS provides a message-threading capability, which organizes your email based on subject -- you click an icon to the left of a message header to see the related messages. That adds more clicking to go through messages, but makes it easier to find the messages in the first place. You can disable threading if you don't like it.
Calendar. You can easily switch calendar views in iOS in the main calendar screen, and you can send invites to other users for non-Exchange calendars by adding an invitee's email address to the appointment. Your invitations for Exchange accounts show up in your calendar as a pop-up; you can accept them there within the full context of your other appointments. For both Exchange and other email accounts, you can open the .ics invitation files in Mail, then add them to the calendar of your choice.
When creating appointments, you can set up to two reminders beforehand at user-specified periods. A nice addition in iOS 5 is the ability to set the default alert intervals for calendar entries -- there are separate settings for regular events (those with start and stop times), all-day events, and birthdays. What iOS lacks is the kind of sophisticated scheduling available on a BlackBerry, such as the first Monday of the month, or every second Monday and Wednesday.
You can set the time zone for each appointment you add -- a very nice tool for those of us who travel across time zones or set phone conferences with people in other time zones and have difficulty translating the time to our current time zone or our calendar's default time zone. (iOS lets you specify a fixed time zone for your calendar or set it to change automatically to the current time zone as you travel.)
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