How do I customize my mail toolbar on my iPad

No.

There is no customisation in iOS Mail.

The only customisation you can make, and this is a global iOS customisation is to make text larger and bolder in the Settings app under Accessibilty and that is that!

If you want email client that is more customisable, there are other email clients available in the iOS App Store.

If you are a Google Mail, Yahoo Mail or Microsoft Hotmail user, here is a almost exact replacement for iOS Mail that adds back all of the missing functionlities in iOS 10 Mail app and adds even more.

The most recent update adds a few more email services, so read the update details!

One user has successfully gotten this to work with their AOL account, if anyone is still using AOL.

Look at this nearly exact email app replacement for iOS Mail.

I really like this email app a WHOLE LOT better than the stock Apple iOS Mail App while still looking very much like the standard iOS Mail App.

This is a real beautiful email app to use and, if you are a large iPad Pro user, then this email app's UI really shines!

Email

Email - mail for Gmail Yahoo iCloud & Hotmail by Easilydo Inc.

https://appsto.re/us/wAla3.i

One of the real nice things about this app as it has a setting to allow the Safari web browser to open right up inside of this mail app.

Which is super convenient and quick!

If anyone can use this iOS Mail app replacement for the iOS 10 version of Mail,

You're Welcome!

What do you mean customize your mail toolbar? What is it that you are trying to do? There really isn't much of a toolbar on the iPad for mail, just the ability to open your different accounts, edit your mail, depending on what area you have open at the time.

Please explain more what it is you are trying to do.

AS Chris stated, you can not customize the mail toolbar, if you are calling the icons in the upper right (flag, move, delete, reply, write) the toolbar. They are static and cannot be moved. You can reorganize the boxes on the left, and add some additional mailboxes if you have an IMAP email account. Tap on the Edit button at the top and take a look at the options.

You can reorder things by dragging up or down on the 3 little horizontal lines on the right.

Tap on the word Edit next to the Mailboxes heading

Tap on New Mailbox at the bottom of the window.

Name the new mailbox.

Select the location of the mailbox - the email account in which you want to add the new mailbox.

Tap Save.

You can only add mailboxes to IMAP email accounts, it does not work with POP accounts.