Actually,
ANd by client I mean Yahoo vs Hotmail vs GMail etc.
Those aren't clients. They are Mail Providers. An E-Mail Client is an application that allows you to view you email In this case the email client is the Mail App on the iPad. Ot the Mail App for one of the Email providers.
The ability to make mailboxes/folders depends not on the Mail Provider itself, but on the type of mail service it offers. IMAP ( Internet Message Access Protocol ) mail has the ability ot create new mailboxes. POP ( Post Office Protocol ) does not offer this ability.
To create new Mailboxes, the mail account needs to be of the IMAP variety.Examples of Mail Providers tha offer this type of Email service are Gmail, Hotmail, Yahoo etc...)
With that said you can move an email from a POP account to an IMAP account's mail box from the iPad by using the little Folder icon.
So you could create a Review folder in one of your IMAP accounts and use that for your emails to review.
If the account is IMAP, tap on the account name in the Mailboxes Pane under the Accounts heading. That will open the Mail account. Tap on the Edit button at the top right corner of the pane, and thne on the New Mailbox button at the bottom right. This will allow you to create a new mailbox folder to put emails into.
You can look for the email you want to move, and using the Folder icon at the top, move it to the folder you just created.