Is it possible to access mailboxes without my old iPad?
Only some of my mailboxes have transferred over to my new iPad. How do I retrieve all the missing mailboxes. I don’t have my old iPad
[Re-Titled by Moderator]
Assuming that your Mail Provider is using IMAP, all of your mailboxes will be restored by adding the Mail account(s) to your new iPad.
Add and remove email accounts on iPad - Apple Support
If setting-up your new iPad using the native transfer process, your mail account settings will usually be automatically transferred to the new device. If instead you are setting-up manually, you will need to add the Mail account(s) yourself.
Alternatively, restoring a backup of your old iPad to the new iPad will restore all settings and data that were included within the backup: