I'm in the process of helping a client move 4 email mailboxes away from @xtra.co.nz, to their own domain, but having issues trying to access their old mailboxes via IMAP.

Logging into Spark webmail works fine for the 4 mailboxes, but I cannot access 2 of them via IMAP.
I need IMAP access so I can move emails from their old mailboxes to new.

I've tried both Outlook and Thunderbird, and both give an error. e.g. Thunderbird says Unable to log in at server. Probably wrong configuration, username or password.
BUT I know the login works, as Webmail works???

I'm using imap.xtra.co.nz, SSL, 993. And send.xtra.co.nz SSL 465

Anyone come across this before, where the Spark webmail login works, but IMAP access doesn't?

Any other suggestions on how to move/copy email out of the old xtra mailbox, to a separate new imap account?

Thanks