iPhone Activesync working, but getting Unexpected Exchange mailbox Server error HTTP status code: [400]. Verify that the Exchange mailbox Server is working correctly.

Are you able to sync iPhone ?

Do the following

Open IIS Manager

Navigate to Websites -> right click on "Default web site" and click on properties.

On the web site tab click on advanced

Under "Multiple identities for this website" click on the default entry and click on edit

IP address: (All Unssigned), TCP port:80, Host Header Value: "Blank".

click ok and restart IIS Admin Service (warning will disconnect clients) to be sure!

Also are you using a external SSL or a home grown SSL. Ive seen issue like this when people use homegrown SSL certificates. If you are, I recommend purchasing a SSL from an external like GoDaddy.com (cos they are cheap there.)

@diggisaur - I am using a go daddy SSL cert.  @alanhardisty I will run through your check list tonight and see if any of these items fix it.  Weirdest thing.  The sync works (mostly).  I get my contacts, calendar and if I go to my inbox, email comes in.  Just getting 9000+ errors a day, plus email is not "pushed" to device.  Only when I go to Inbox and it connects do I get new emails.  Kind of defeats the purpose of the whole thing.

Is your activesync on the phone set to As Items Arrive or something else?
Click on Start, Programs, Activesync, Menu, Schedule.
Check Peak and Off-Peak items and make sure they are set to As Items Arrive.

Sorry - that's for Windows phones.   Please ignore my last comment.

Click on Settings, Mail Contacts Calendars, Fetch New Data.  Is Push set to On?

@alanhardisty Yes, it is.  Fetch is set to Manual.  

Okay - Can you please check the server settings under Exchange System Manager, Global Settings, Mobile Services.
Right-click Mobile Services and choose properties.
What is ticked?
Under Device Security - what is ticked?
 

Can you attempt to factory reset one phone and just configure ActiveSync first to see what happens? Might be some conflict with the existing fetch settings.

Well, I got the event ID 3005 to stop.  I went through all the check lists provided by (@alanhardisty / 09/09/09 01:06 PM, ID: 25293950) but was still get the event 3005 every few seconds for both iPhones showing up in the log.  So, I rebooted the server.  Now, I no longer get 3005 every few seconds and it has been replaced by 3015 (IP-based AUTD failed to initialize because...) every few seconds.  Error pasted below.  Researching now...

Event Type:      Error
Event Source:      Server ActiveSync
Event Category:      None
Event ID:      3015
Date:            9/11/2009
Time:            2:53:18 PM
User:            N/A
Computer:      <COMPANYNAME>
Description:
IP-based AUTD failed to initialize because the processing of notifications could not be setup.  Error code [0x80004005].  Verify that no other applications are currently bound to UDP port [2883], or try specifying a different port number.

Please stop the DNS server service and then restart it.  This quite often grabs the AUTD port.
There is a KB article to reserve ports to stop this happening - I'll search for it and come back in a sec.

Okay - not a KB article - but this will stop it from happening again:
http://blogs.technet.com/sbs/archive/2008/07/17/some-services-may-fail-to-start-or-may-not-work-properly-after-installing-ms08-037-951746-and-951748.aspx 

Okay, found a fix for 3015 on this site by restarting DNS and now the 3005 is back.  I am out of ideas and have run through all that I have recieved here so far.  iPhone is a blast!!!

Have you checked the settings detailed in the KB article at the end of my initial posting?
http://support.microsoft.com/kb/817379
Extract from: http://social.technet.microsoft.com/Forums/en-US/exchangesvrmobility/thread/66a33f3a-d945-42e2-ac80-3e4244f9b258
Sure enough, adding a IP Address, TCP Port, Host header value identity entry with the host header left blank, that is (Default, 80,<empty>) under the IP Address Advanced section for the Default Web Site properties for my IIS server definately makes a difference.
Do you have the above setting?

i have seen many such cases where even though mobile device is syncing we still get 3005 upon sync. and most of the cases were resolved by updating Massync.dll, setting exclusions on AV,removing Host header from Default website ,remove ip from Default web site and make it run under all unassigned.removing IP from exchange-oma if it is there. sometimes even reseting the virtual directories (method 2 KB 883380).

error 3005 is very generic error for ActiveSync which doesnt always mean issue with ActiveSync.

To obtain latest hotfix for Massync.dll
http://support.microsoft.com/kb/957191

I have checked and / or implemented all recommendations still with no resolve to the HTTP 400 error.  I have now disabled push as the constant back and forth was draining the battery on the iPhone at an alarming rate (10% every hour).  

I may have to go back to the drawing board on this one.  

I can make the error stop by turning the Push function off on the iPhone.  As soon as I turn it back on, the Event ID 3005 errors start back up with the HTTP 400 errors.  They come at a rate of about 4,500 per device per day.  Apple was absolutely no help and basically indicated it had to be an issue on the Microsoft side.  Any suggestions other than those above?

Can you try one more thing before you have no hair left please!
Under Exchange System Manager> Global Settings> Mobile Services, right-click and choose properties.
Click on Device Security on the General Tab.
Click on Exceptions on the Device Security Settings Window.
Add your user account details to the exceptions.
Try syncing again and see if you get the errors again please.
Fingers crossed.
 

No dice, the dreaded HTTP 400 errors began as soon as I turned push back on.  I will say this, before this last suggestion you had, I had not enabled the "Enforce password on device" option.  I enabled it, then added my user account to the exceptions.  Same issue occurs.  

The other settings from this area are all checked:
Enable user initiated synchronization
Enable up-to-date notifications via SMTP and Text Messaging
- Enable notifications to user specified SMTP addresses
Enable Direct Push over HTTP(s)

Any issues you can see with those settings?  Again, all are presently enabled (checked)

Have you downloaded the free iPhone App from the iStore - Activesync Tester - and tried running it?
Might throw up something - might not.  Nothing ventured - nothing gained!

Yes, I have and it passes with flying colors.  Originally, the issue I had was I could not get email to connect at all, until I loaded SP2 for Exchange.  After I loaded that, the Activesync Tester worked and I could add and use my Exchange account.  Just Push doesn't play well in the sandbox.

Have you wiped the iPhone back to factory settings and set it up again yet?
If not - is that a viable option?

I have done so a couple of times, plus get this on more than one iPhone.  Could do the wipe again.  I am also going step by step through your check list again.  BlackBerry and Windows Mobile never gave me this much grief.  

No - my iPhone just got on with it!
What OS version are you using on the iPhones?

The latest update which is out, 3.1 plus the recent AT&T MMS carrier update.  Issue persisted through all.  Started at 3.0 at the beginning of the month.

Okay, just walked through your checklist again.  The last item states verifying the ASP.NET is set to 1.1 for all Vir Dir.  How / where do I do this verification?

On the default website virtual directories.  There will be a tab, which presumably appears if yoy have asp.net 2, so naturally, this suggests you don't have it installed.

This then leads on to the question - do you have any Windows updates that have not been installed, critical as well as recommended?

Please check and install, then reboot, check again and repeat until there are none left.

Working the patches, etc. now.  There are some outstanding.  Will advise when completed.

Have fun!

No dice. All patches and recommended updates and their patches applied. Nothing pending now, yet the error persists.

Okay - re-read the question!  All of it!
HTTP 400 is a Bad Request error.
So the error stops happening when the Push feature is turned off.
If you manually sync - does the error appear?
Does push on both iPhones causes the error?
Have you got a Windows Mobile phone you can test to see if the push works without errors?  This would help rule out the server or the iPhones as the potential cause.
What router do you have protecting you from the evil world outside? (Make / Model please)