Where are the ZOHO IMAP emails stored

Where are the ZOHO IMAP emails stored

I am a little bit confused after doing a migration via IMAP into my new ZOHO account. When I check the available storage it shows 100% free meaning no emails have been stored yet. However, I did the migration successfully. Does this mean that ZOHO simply accesses the emails via IMAP from my old server, or are they now stored at ZOHO?

I need to know as I am about to deleted the old files at my previous hosting company but I have to 100$ sure the emails will not disappear once I do that