Migration from Fastmail

Hi there-

No matter what I do, I can't seem to migrate from Fastmail to Zoho.

I've triple checked my server details, and I'm using the exact same account details (adding manually) that i use to login to my email now but it keeps coming back saying "Authentication details provided was rejected by Source server. Kindly provide..."

Anyone have any success migrating from fastmail? What am I missing?