I managed to locate a fix and have done what it says in here and it is now 
sorted. Seems to be a bug confined to El Capitan.

http://solutionowl.com/mac-mail-account-already-exists-capitan/

Regards


Pete

> On 13 Sep 2017, at 5:07 pm, Peter Crisp <petercr...@westnet.com.au> wrote:
> 
> Hi there, my wife finally got around to rationalizing her Inbox down to <2000 
> unread emails and so given my frequent hassling to get her to migrate to 
> IMAP, she gave the all clear for me to convert her from POP to IMAP Mail 
> account. I had created her IMAP account (but not enabled) some years ago when 
> I made the change myself but these things take time. When given the all 
> clear, I followed the instructions below. Before starting I had deleted the 
> IMAP account created some years ago (the one that had never been enabled) as 
> I was unsure if this would in fact be ok but figured I would be creating an 
> account per step 5 in the instructions below, so should be ok I thought. So I 
> went through Steps 1-4 no problem though it took quite some time to Move the 
> Sent items (~20,000) to a temp folder in the "On My Mac" space. Anyway when 
> the Inbox and Sent items move was completed, I tried to create the IMAP 
> account - I get a message which says "the account already exists". Which I 
> think is Mail's way of saying, "I checked the Server (Westnet) and there is 
> already an account there with those details". The POP account is certainly 
> still available though unticked the "Enabled" button. I suspect the server 
> still has the remnant stub of the IMAP account I created years ago and had 
> subsequently deleted. So my issue is how do I re-create the IMAP account on 
> the MacBook (El Capitan 10.11.5). Any tips welcomed. I rang Westnet who 
> didn't really offer any help but I am surprised and think the issue is at the 
> Westnet server. 
> 
> Regards
> 
> 
> Pete
> 
> Begin forwarded message:
> 
>> From: Peter Crisp <petercr...@westnet.com.au>
>> Date: 11 September 2017 at 12:54:51 pm AWST
>> To: Peter Crisp <petercr...@westnet.com.au>
>> Subject: Fwd: How to convert a POP email account to IMAP
>> 
>> 
>> 
>> Regards
>> 
>> 
>> Pete
>> 
>> Begin forwarded message:
>> 
>>> From: Peter Crisp <petercr...@westnet.com.au>
>>> Date: 15 October 2015 at 9:58:27 pm AWST
>>> To: Tim Mills <timsph...@westnet.com.au>
>>> Subject: Fwd: How to convert a POP email account to IMAP
>>> 
>>> I think it was this one.
>>> 
>>> Regards
>>> 
>>> 
>>> Pete
>>> 
>>> Begin forwarded message:
>>> 
>>>> From: Peter Crisp <petercr...@westnet.com>
>>>> Date: 15 October 2015 at 8:07:07 AM AWST
>>>> To: Tim Mills <timsph...@westnet.com.au>
>>>> Subject: Fwd: How to convert a POP email account to IMAP
>>>> 
>>>> Hi Tiffy, what email account type do you operate with? 
>>>> 
>>>> See below thread.
>>>> 
>>>> Regards
>>>> 
>>>> Pete
>>>> 
>>>> 
>>>> Begin forwarded message:
>>>> 
>>>>> From: Peter Crisp <petercr...@westnet.com.au>
>>>>> Date: 14 October 2015 at 9:20:38 PM AWST
>>>>> To: WAMUG <wamug@wamug.org.au>
>>>>> Subject: Re: How to convert a POP email account to IMAP
>>>>> 
>>>>> Hi Rob, Neil and Daniel et al, the ‘Option’ key did the trick. the 
>>>>> IMAP/POP option presents when you hold the Option key down at the 
>>>>> ‘create’ stage. It went very smoothly from there. Once created it 
>>>>> immediately synchronised around 3,500 emails from the server (mostly Sent 
>>>>> items) - it took just a few minutes. I dragged back the temporarily 
>>>>> created ‘Inbox’ to the real Inbox.
>>>>> 
>>>>> Then I deleted the accounts on my iPhone and iPad and then recreated 
>>>>> there, again straightforward update.
>>>>> 
>>>>> It seems all good now.
>>>>> 
>>>>> I will see how it behaves over the next few days and then recommend to my 
>>>>> wife potentially.
>>>>> 
>>>>> Thanks for the help here folks.
>>>>> 
>>>>> Regards.
>>>>> 
>>>>> Pete...
>>>>> 
>>>>> 
>>>>> 
>>>>>> On 14 Oct 2015, at 7:10 pm, Rob Phillips <r.phill...@iinet.net.au> wrote:
>>>>>> 
>>>>>> Hi Pete
>>>>>> 
>>>>>> Daniel's suggestion is the best way to go.
>>>>>> 
>>>>>> However, note that I suggested (perhaps not very clearly) that you 
>>>>>> recreate your old email 'account' in mail.  The important thing to 
>>>>>> recognise is that your email account with Westnet still exists. It's 
>>>>>> just that the connection to that account in the Mail email browser is 
>>>>>> being changed. Unfortunately, Mail calls this an 'account', so it gets 
>>>>>> confusing.
>>>>>> 
>>>>>> Good luck
>>>>>> Rob
>>>>>> 
>>>>>>> On 14/10/2015 6:02 pm, Peter Crisp wrote:
>>>>>>> Hi Rob, the issue is I want to retain my existing email address. To 
>>>>>>> change that means a massive undertaking in alerting the many places 
>>>>>>> where my email address is embedded - Apple store, wine sellers just to 
>>>>>>> name a couple. So the point being I only want to change the account 
>>>>>>> type but keep the rest unchanged.  Daniel has chimed in with a 
>>>>>>> suggestion which I will try tonight. I'll advise the group of success 
>>>>>>> (hopefully).
>>>>>>> 
>>>>>>> Regards
>>>>>>> 
>>>>>>> Pete
>>>>>>> 
>>>>>>> On 14 Oct 2015, at 2:54 PM, Rob Phillips <r.phill...@iinet.net.au> 
>>>>>>> wrote:
>>>>>>> 
>>>>>>>> Hi Pete
>>>>>>>> I think the problem might be in trying to use the same email address.
>>>>>>>> 
>>>>>>>> A possible way forward is (with lots of backups and care - including 
>>>>>>>> downloading recent emails onto a second device)
>>>>>>>> create a new email address - gmail - or even better a second westnet 
>>>>>>>> address associated with your account
>>>>>>>> try to create an IMAP account for this
>>>>>>>> Move your old messages into new new mailbox
>>>>>>>> try the disable/ enable thing again. Maybe you can change the server 
>>>>>>>> settings now.  Failing this, delete that old mailbox and create a new 
>>>>>>>> IMAP one.
>>>>>>>> Copy your old messages back.
>>>>>>>> I hope this helps
>>>>>>>> Rob
>>>>>>>> 
>>>>>>>>> On 14/10/2015 7:55 am, petercr...@westnet.com.au wrote:
>>>>>>>>> Hi Rob, thanks for the suggestion. The process in Mail to go from POP 
>>>>>>>>> to IMAP requires establishment of a new account, you;re unable to 
>>>>>>>>> just change the server address as the Account Type (POP) is superior 
>>>>>>>>> to that. Unfortunately I am unable to get past the first dialogue 
>>>>>>>>> where entry of the account detail (Name, email address and password) 
>>>>>>>>> are required before getting to selecting IMAP/POP and server details. 
>>>>>>>>> That is the problem I have. I have altered the Name and password of 
>>>>>>>>> the account from the disabled account detail (no point changing the 
>>>>>>>>> email address as the old address is MY address) and still no joy. 
>>>>>>>>> Might have to ring Apple or chat.
>>>>>>>>> 
>>>>>>>>> Thanks.
>>>>>>>>> 
>>>>>>>>> Pete.
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> ----- Original Message -----
>>>>>>>>> From:
>>>>>>>>> wamug@wamug.org.au
>>>>>>>>> 
>>>>>>>>> To:
>>>>>>>>> <wamug@wamug.org.au>
>>>>>>>>> Cc:
>>>>>>>>> 
>>>>>>>>> Sent:
>>>>>>>>> Tue, 13 Oct 2015 22:03:09 +0800
>>>>>>>>> Subject:
>>>>>>>>> Re: How to convert a POP email account to IMAP
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> I use Thunderbird not Mail, but have you changed the mail server 
>>>>>>>>> address? It should be something like imap.westnet.com.au, instead of 
>>>>>>>>> pop.westnet.comau.
>>>>>>>>> 
>>>>>>>>> Hope this helps.
>>>>>>>>> Rob
>>>>>>>>> 
>>>>>>>>> On 13/10/2015 8:13 pm, Peter Crisp wrote:
>>>>>>>>> Hi, have any of you done the below process of moving from a POP Mail 
>>>>>>>>> account to an IMAP account? I tried again and I get the same result 
>>>>>>>>> as below here. Not able to Create a new account as Mail simple 
>>>>>>>>> re-enables the account I disabled. 
>>>>>>>>> 
>>>>>>>>> Regards
>>>>>>>>> 
>>>>>>>>> Pete...
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> On 12 Oct 2015, at 9:16 pm, Peter Crisp <petercr...@westnet.com.au> 
>>>>>>>>> wrote:
>>>>>>>>> 
>>>>>>>>> Hi folks, I am getting a moment to make this change to my westnet 
>>>>>>>>> account from POP to IMAP. The text (in different font) below is that 
>>>>>>>>> from a previous thread on the topic back in Feb 2014. I am running 
>>>>>>>>> V8.2 Mail and OS X 10.10.5. Maybe Mail is a little different now and 
>>>>>>>>> the instructions altered or maybe I am doing something wrong. I’ve 
>>>>>>>>> made the copy of Inbox and Sent items into temp folders “On My MAC”. 
>>>>>>>>> Then I uncheck the Enable button in my current account. No problem to 
>>>>>>>>> here. 
>>>>>>>>> 
>>>>>>>>> Then I choose Mail > Add Account and select “Add other Mail account”. 
>>>>>>>>> This is where I am unsure. I used the same account name, email 
>>>>>>>>> address and password (as I am trying to change to IMAP) as current 
>>>>>>>>> account and once I select Create, it simply enables the existing 
>>>>>>>>> account which I just disabled. Perhaps this is not unexpected. I 
>>>>>>>>> tried a different password and account name (same email address) - 
>>>>>>>>> same thing - re-enables the existing disabled account.
>>>>>>>>> 
>>>>>>>>> What am I doing wrong or is the procedure different in 8.2?
>>>>>>>>> 
>>>>>>>>> For the moment I have re-enabled my Mail so I can still use it for 
>>>>>>>>> this.
>>>>>>>>> 
>>>>>>>>> Regards
>>>>>>>>> 
>>>>>>>>> Pete.
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> Hi Laura,
>>>>>>>>> 
>>>>>>>>> I have cc'd your iinet &  Laura Webb <elja...@me.com> addresses, so 
>>>>>>>>> hopefully 
>>>>>>>>> you receive at least one of them.
>>>>>>>>> Step 2. is VERY IMPORTANT: 
>>>>>>>>> To avoid any potential disasters, save all the messages you have on 
>>>>>>>>> your Mac to 
>>>>>>>>> a different mailbox before you convert to IMAP.
>>>>>>>>> 
>>>>>>>>> How to convert a POP email account to IMAP
>>>>>>>>> Most email providers let you choose between two ways to get your 
>>>>>>>>> messages. You 
>>>>>>>>> can have a POP (Post Office Protocol) account that downloads all your 
>>>>>>>>> messages 
>>>>>>>>> to your Mac, iPhone or iPad. Or, you can have an IMAP (Internet 
>>>>>>>>> Message Access 
>>>>>>>>> Protocol) account that leaves some or all of your messages up on the 
>>>>>>>>> mail 
>>>>>>>>> server so you can access them from any device. For example, with a 
>>>>>>>>> POP account, 
>>>>>>>>> if you check email on your iPhone, the messages downloaded there will 
>>>>>>>>> only be 
>>>>>>>>> there; when you get back to your Mac, you won’t see them in your 
>>>>>>>>> inbox. With 
>>>>>>>>> IMAP, they’ll be on all your devices.
>>>>>>>>> 
>>>>>>>>> If you’ve been using a POP account, but have grown frustrated with 
>>>>>>>>> inaccessible 
>>>>>>>>> messages, you can usually convert your account to IMAP. Most hosting 
>>>>>>>>> companies 
>>>>>>>>> and providers offer both options. However, making this conversion 
>>>>>>>>> requires that 
>>>>>>>>> you take precautions so you don’t lose any messages. Here’s how to 
>>>>>>>>> make the 
>>>>>>>>> switch safely:
>>>>>>>>> 
>>>>>>>>> 1. Check with your email host
>>>>>>>>> 
>>>>>>>>> If you manage your email account yourself, check on your hosting 
>>>>>>>>> company’s 
>>>>>>>>> dashboard to make sure you can use IMAP. (It’s very rare that this is 
>>>>>>>>> not 
>>>>>>>>> available.) If you get email through your business or school, check 
>>>>>>>>> with the IT 
>>>>>>>>> department. Find out what settings you will need to access your email 
>>>>>>>>> as an 
>>>>>>>>> IMAP account, and whether anything needs to be changed on the server 
>>>>>>>>> side. If 
>>>>>>>>> you use Gmail, this help page will tell you how to change your Gmail 
>>>>>>>>> account 
>>>>>>>>> from POP to IMAP. (You have to enable IMAP on the Gmail website.)
>>>>>>>>> 
>>>>>>>>> 2. Protect the message you’ve already downloaded
>>>>>>>>> 
>>>>>>>>> To avoid any potential disasters, save all the messages you have on 
>>>>>>>>> your Mac to 
>>>>>>>>> a different mailbox before you convert to IMAP.
>>>>>>>>> 
>>>>>>>>> IMAP stores some or all of your email on the mail server. The problem 
>>>>>>>>> is, this 
>>>>>>>>> means that any folders that are on your Mac may get overwritten, or 
>>>>>>>>> deleted, if 
>>>>>>>>> you don’t save their messages before making the switch. So, in Mail, 
>>>>>>>>> choose 
>>>>>>>>> Mailbox -> New Mailbox, and, with the location as On My Mac, create a 
>>>>>>>>> new 
>>>>>>>>> mailbox. Call it something like My Old Inbox. When you’ve done this, 
>>>>>>>>> move all 
>>>>>>>>> the messages currently in your Inbox to this folder. Do the same for 
>>>>>>>>> your Sent 
>>>>>>>>> Mail folder, and any Drafts you may have. Once this is done, you can 
>>>>>>>>> change the 
>>>>>>>>> settings for your email account.
>>>>>>>>> 
>>>>>>>>> 3. Find your IMAP settings
>>>>>>>>> 
>>>>>>>>> Either your hosting company, your IT department, or your email 
>>>>>>>>> provider will 
>>>>>>>>> provide the settings you need to switch your account over to IMAP. 
>>>>>>>>> For example, 
>>>>>>>>> for Gmail, you will need to know the new Incoming Mail Server 
>>>>>>>>> (imap.gmail.com), 
>>>>>>>>> but your outgoing mail server won’t change. In most cases, you will 
>>>>>>>>> just need 
>>>>>>>>> to change the prefix in the Incoming Mail Server setting from pop to 
>>>>>>>>> imap. 
>>>>>>>>> Write these down. You'll need them in Step 5.
>>>>>>>>> 
>>>>>>>>> 4. Disable your POP account
>>>>>>>>> 
>>>>>>>>> In Mail, choose Preferences -> Accounts, click on the name of your 
>>>>>>>>> POP account, 
>>>>>>>>> then on Advanced, then uncheck Enable this account.
>>>>>>>>> 
>>>>>>>>> 5. Create a new IMAP email account
>>>>>>>>> 
>>>>>>>>> In Mail, you can’t change an email account from POP to IMAP. Instead, 
>>>>>>>>> you need 
>>>>>>>>> to create a new account. 
>>>>>>>>> 
>>>>>>>>> Choose File -> Add Account, and enter the information that you 
>>>>>>>>> obtained in step 
>>>>>>>>> 3 above.
>>>>>>>>> 
>>>>>>>>> 6. Check your email
>>>>>>>>> 
>>>>>>>>> If you entered the correct information, your new IMAP account should 
>>>>>>>>> work right 
>>>>>>>>> away. You will see an Inbox folder that is on the email server. Copy 
>>>>>>>>> the 
>>>>>>>>> messages you stored in your old inbox folder in step 2 to that 
>>>>>>>>> folder, and they 
>>>>>>>>> will be copied to your email server, so they will now be accessible 
>>>>>>>>> from all 
>>>>>>>>> your devices. Do the same for any sent messages or drafts.
>>>>>>>>> 
>>>>>>>>> Now that you have an IMAP account on your Mac, use the same settings 
>>>>>>>>> to access 
>>>>>>>>> your messages on another Mac, or on your iOS device(s).  On your iPad 
>>>>>>>>> or 
>>>>>>>>> iPhone, enter the necessary information in the Settings app, in the 
>>>>>>>>> Mail, 
>>>>>>>>> Contacts, Calendars section.
>>>>>>>>> 
>>>>>>>>> If you create more folders to store messages you’ll see them on all 
>>>>>>>>> your 
>>>>>>>>> devices. To do this, choose Mailbox -> New Mailbox, then in the popup 
>>>>>>>>> menu at 
>>>>>>>>> the top of the dialog box, choose the name of your email account. The 
>>>>>>>>> folder 
>>>>>>>>> will be created on your mail server, and any messages you store in 
>>>>>>>>> that folder 
>>>>>>>>> will sync to all your devices.
>>>>>>>>> 
>>>>>>>>> IMAP is much better for managing email across multiple devices. 
>>>>>>>>> Change your old 
>>>>>>>>> POP accounts to IMAP now so you can have access to your email at all 
>>>>>>>>> times.
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> Cheers,
>>>>>>>>> Ronni
>>>>>>>>> 
>>>>>>>>> 17" MacBook Pro 2.3GHz Quad-Core i7 “Thunderbolt"
>>>>>>>>> 2.3GHz / 8GB / 750GB @ 7200rpm HD
>>>>>>>>> 
>>>>>>>>> OS X 10.9.1 Mavericks
>>>>>>>>> Windows 7 Ultimate (under sufferance)
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>> 
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