Given the choice most users, in my experience, want a migration that means the minimum work for them. So a migration that preserves their existing emails and folders is the ideal. Anything that involves having two systems live at the same time is likely to confuse some.

Having had to migrate thousands of users from one email system to another, and from one server operating system to another, I find that it is very difficult to please all the people all the time. In the current situation I would say that it was more likely that the fault lies with Sky/Yahoo than with Google.

In reply to a post by Lethe:

If a user deletes an email it should be deleted? Yes or no?

Moved to Trash / Deleted Items / Bin is the modern day interpretation of "delete"

You then have to "empty deleted items" or the like to do what I suspect you want to happen.

My Gmail Bin has 300 items from the last 2 weeks, and I know it's there and empty it.

Users are certainly likely to be less happy with material disappearing unexpectedly than with "deleted" stuff reappearing. Of course, users are never truely happy whatever happens.

i guess users are not aware that on google mail unless you change the default settings pop3 does not delete the email from the server unless you go onto the google mail or sky mail as it is and change the setting

so when sky moved all the mail from Google to yahoo, all mail gets re-downloaded again if using pop3 as the mail is now on yahoo servers

only other issue now is all sky users can now be used to spam any one due to an hole in yahoo network still ( http://thenextweb.com/insider/2013/03/06/despite-its... ,ignore comments from users who have had accounts stolen/locked as more likely they have given there password away somewhere)