What would you say to an IT professional

[Wainuitech]

But I would say where he seriously went wrong was in doing a 'move' operation rather than a 'copy' - regardless of source/destination, if it's important stuff I will always copy first, verify it arrived correctly, only then delete, or rename the folder old_xxxxxx.

That's the Key. Should  not 100% Trust Cloud storage. 

Where it gets real fun is if web mail is all that's used, cant beat the good old install a program, Outlook, Thunderbird, EMClient etc.

About the emails -- thats where IMAP can be  dangerous, if it were a .pst file stored locally then you can always run a recovery and get things back. 

Just had to do a recovery from a customer, outlook at 73GB   ( enlarged the default .pst size a couple of times now,  yes large and been warned multiple times)  Ran a email recovery, took a couple of days to scan the file, but got mails going back over 15 years. 

he tried to move his extra mails to Microsoft Outlook

Done a couple now for people, easy as regardless of IMAP or .pst.  If .ost 1st make a backup - export the file and save as .pst then you have a full working fully safe backup.

Simply create the new account, right click the folder - Copy - To new account - the IMAP will populate, may take some time but it will do it. Once copied you can remove the original.  ALWAYS copy / Paste, not Move/Cut  Something goes wrong and you can  lose the lot. Copy fails - You swear, blame the cat and try again