If you currently are not using an IMAP based email service converting can be a bit of a an "un-learning" experience. The common/traditional methods of Internet email reception (which has been outdated since at least 2003) has forced humans in to limited usage patterns that will change when converting to the more-better IMAP.Would then need to work-around with "Leave on Server" - SUX Then, odd issues. And Sent Item sonly exist on the one device. Each device gets it's own copy; they all have to play nice (leave on server) so they get along; Generally one becomes the archive (Desktop) Sent Items are unique to each Device
The traditional method, sometimes referred to POP or POP3 stores a copy of a message on every computer that access the mailbox. If you have multiple devices, each has to be configured to "play-nice" and not destroy message before the other devices can see them (sometimes called "Leave on Server").
POP3 is Post Office Protocol Version 3 - and it operates in similar fashion. Messages are delivered to your virtual "Post Office" where they wait until you recieve them.
There are many reasons we don't like this method.
IMAP (Internet Mail Access Protocol) is the improved method of mail access. The key factor is that messages are always stored on the central system, and each device accesses this one shared mailbox. Activity in the mailbox from one device (sending, reading, deleting) is reflected on all the other devices as well.
How IMAP Works and also some notes on Migrating.
Here are the general instructions that Google provides: http://support.google.com/mail/bin/answer.py?hl=en&answer=78799 and there are some device/system specific instructions here: http://support.google.com/mail/bin/answer.py?hl=en&answer=75726