IMAP promise to existing free users doesn't seem to be kept

IMAP promise to existing free users doesn't seem to be kept

I've emailed support about this but had no reply. My phone needed a software update and I've had to reload my two Zoho emails. One free (and well over ten years old) and one paid. The phone has accepted the IMAP credentials for the paid account but not the free one. Why? I'm loading to Outlook for Android and Blackberry Hub. It's the same IMAP details for both my accounts. How are you "marking" old free accounts so IMAP still works on them.