Server rejecting credentials when setting up a new email account


I have an account at owncube and am trying to set-up an email address using a different domain. Owncube support are saying as it is third party they won’t support. Steps take so far:

  1. On the same computer log into webmail using password - it worked
  2. On the same computer set up the email account in Thunderbird using the same imap credentials - it worked
  3. On new ip address and different computer repeat steps 1 and 2 - both worked
  4. On both computers and different ip addresses set-up fails.
  5. Check with email hosting company all settings are correct.
  6. Tried automatic and manual set-up - failed

Fails with the message Error while creating the account: Creating account failed: Server denied authentication.

Any ideas please?