The use of .pst files is no longer required for storage of archive emails as Office 365 allows up to 50Gb of storage - for most of you this is 150x more than the old Exchange 2003 limits.
This FAQ is designed to assist you in copying email in your .pst files to your Office 365 mailbox.
You must use Outlook to copy email from .pst files
First you need to remove the .pst files from Outlook
^ Right click on the offline folder where listed in Outlook and click on Close 'Name of file'. Close Outlook and re-open it again
Decide where you want to put the email you're importing
You may choose to create a new folder, or to import the email back into the file structure it was exported from. The former more useful if you have created your own system for filing mail in offline folders or wan to keep the old email filed separately. The latter is most useful if you've used auto archive.
If you're going with a new folder, create it now. To keep the existing structure, you can skip this step.
Now import the email from the .pst file to Office 365.
If you're going with a new folder, select it now.
^Click on 'File'
^Click on 'Open'
^Click on 'Import'
^Select 'Import from another Program or file' and click 'Next >'
^Select 'Outlook Data File (.pst)' and click 'Next >'
^Outlook should open your .pst area automatically, however if it doesn't the path is \\pst\pst$\%username% which takes me to...
^Here I have a number of .pst files. Do NOT try to upload any files starting with 'SharePoint Lists'. The only file in my .pst store with any email in it is the one titled 'AndyToddOfflineEmail-DoNOTdelete.pst'. Obviously your may have different names.
^ Files which show their size as 265 KB or smaller are likely to be empty and you can usually ignore them. As an example, here Andy has 2 files to copy and one to ignore.
Select the relevant file and click 'open'
^ Select the 'Replace duplicates...' option and click 'Next >'
^ At this point if you're importing into a new folder, choose 'Import items into the current folder'.
If you want to keep the existing structure of the offline folder, choose 'Import items into the same folder in:'. You should check Outlook is going to import the mail into your mailbox, however you can choose any mailbox you have access to.
^ Go make coffee or something (if your .pst files are large, then you might be able to go and pick your coffee directly off the bush and still get back before the transfer finishes), this may take a while...
Now repeat for all of the other .pst files you have used.
Finally check your folders look correct
As you can see the folder has appeared in the mailbox folder tree, but I do need to check the folders and sizes look correct.
and delete your old files...
Your .pst files are located at \\pst\pst$\%username%. Just delete them. Simples.