New mailboxes not alphabetizing on MapBook Air

Apple Mail in macOS Tahoe does not automatically alphabetise every mailbox in the sidebar in all circumstances.

The sidebar can contain different sections, and their ordering behaviour differs:

• Favourites: Mailboxes can be arranged manually.

• Mailboxes under an email account: Their order may reflect the account's hierarchy or manual arrangement rather than automatically sorting every new mailbox alphabetically
• On My Mac: Local mailboxes may also require manual organization.

So, if a newly created mailbox appears at the bottom instead of in its alphabetical position, that does not necessarily mean something is wrong with your MacBook Air.

Apple's Mail User Guide for macOS Tahoe specifically explains how to reorder mailboxes by dragging them; it does not document an automatic alphabetising command. 

One important question: Before the new mailboxes stopped alphabetizing, did your existing mailboxes sort themselves automatically from A to Z whenever you created a new one? 

"New mailboxes not alphabetizing on MapBook Air: Why are new mailboxes not being alphabetized on my MapBook Air, macOS Tahoe version 26.6.2?"

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Alphabetizing Mailboxes in Mail on a Mac:

By default, Mailboxes are sorted as you create them, directly on the bottom. So, are you saying is it WAS alphabetical, and no longer is after an update? You need to drag and drop them in such order.

1. Go to: View menu
2. Hover Over: Sort By >
3. Select: Subject

It was "normal" for a new mailbox to appear at the bottom of the "ON My Mac" list when it was first created. As soon as Mail was reopened it would be alphabetical. Creating a "sub" mailbox under an existing mailbox always immediately appeared alphabetically under the existing mailbox. Suddenly, while dragging an existing mailbox titled "Airlines" (which had 8 "Sub" mailboxes for various airlines) to another location, I inadvertently released the mailbox I was dragging. Its new position was under the letter J on the "On My Mac" side bar. I have been unable to drag it any place else. All new mailboxes are now being list alphabetically beneath the "Airlines" mailbox under the letter "J" in the side bar and I cannot move them.

???