Hi Brett,

I think you are almost there, but not quite :-)
If I understand you correctly you are trying to create a subfolder within an 
existing folder in Mail.

Try this:

1. Select "New Mailbox" to create a custom mailbox folder. 
2. Click the tab next to "Location" and select a destination in Mail to save 
your folder. 
3. To create a subfolder within an existing folder in Mail, click it as your 
location. 
4. Type a name for your folder into the field and click the "OK" button.

Cheers,
Ronni

13-inch MacBook Air (April 2014)
1.7GHz Dual-Core Intel Core i7, Turbo Boost to 3.3GHz
8GB 1600MHz LPDDR3 SDRAM
512GB PCIe-based Flash Storage

macOS High Sierra 10.13.6


> On 11 Jan 2019, at 8:27 am, Brett Curtis <br...@magicleanwa.com.au> wrote:
> 
> Mojave 10.14.1
> Mail 12.1
> 
> Regards
> Brett Curtis
> br...@magicleanwa.com.au <mailto:br...@magicleanwa.com.au>
> 04 0354 0354
> 
> 
> 
>> On 10 Jan 2019, at 4:17 pm, Ronni Brown <ro...@mac.com 
>> <mailto:ro...@mac.com>> wrote:
>> 
>> Hello Brett,
>> 
>> First before I can offer any help - What version of  macOS and Apple Mail 
>> are you using?
>> The versions of Mail in Mojave, High Sierra, iOS 12, and iOS 11 contain new 
>> features and design changes. 
>> 
>> Cheers,
>> Ronni
>> 
>> 13-inch MacBook Air (April 2014)
>> 1.7GHz Dual-Core Intel Core i7, Turbo Boost to 3.3GHz
>> 8GB 1600MHz LPDDR3 SDRAM
>> 512GB PCIe-based Flash Storage
>> 
>> macOS High Sierra 10.13.6
>> 
>> 
>>> On 10 Jan 2019, at 12:31 pm, Brett Curtis <br...@magicleanwa.com.au 
>>> <mailto:br...@magicleanwa.com.au>> wrote:
>>> 
>>> I’ve just returned to using Mail after a long break.
>>> My question is regarding opening a new sub folder to my business email.  
>>> Although I did this successfully a few months ago, I’m now having problems.
>>> I click on the mailbox and choose “New mailbox”
>>> I name the new one and press OK.  It appears for a second or so at the 
>>> bottom of the mailbox section, then disappears.
>>> If I try again, using the same name, it won’t let me, saying a mailbox of 
>>> that name already exists.
>>> I can create a subfolder mailbox to one of the existing 3 sub mailboxes but 
>>> cannot move them up in the hierarchy.
>>> 
>>> tia
>>> 
>>> Regards
>>> Brett Curtis
>>> br...@magicleanwa.com.au <mailto:br...@magicleanwa.com.au>
>>> 04 0354 0354
>> 
>> 


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