Copy large amount of mail from one IMAP account to another (Outlook user)

Question

One of my clients wants to consolidate their domains into one umbrella domain, which also means that they want to COPY all mail from multiple IMAP accounts, to their new single IMAP account.

It is easy enough to copy folders/mail from account to account in Outlook, but I have a feeling that attempting this with tens of thousands of items might prove problematic and/or time consuming.

Is there an easier, more reliable way?

Thanks