GuyinCR Said:
"Custom Domain email limits: iCloud now supports custom domains, but limits email addresses to3 . I now have five email addresses in one custom domain, used by the 2 people in the family. Is there a way to carry that over to iCloud? The domain has been hosted elsewhere for years."
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Merge These Accounts:
With 5 accounts, yore only option is to merge them, manually. You would export the eMails form the eMail provider's site, and then import the eMails to iCloud. If the batches are too large, then try exporting them in smaller batches.
Use a Computer:
Perform this using a computer's it could be more user friendly doing so. Go Here: Import or Export Mailboxes in Mail on Mac - Apple Support