We have recently migrated our exchange users from a 2010 exchange to 2013 exchange. 

Users have Outlook 2010 and recently some of these users have been having issues when sending emails. The emails will sit in the Outbox and not send. 

We currently have cached exchange mode enabled (would prefer to keep this enabled.) 

Currently the resolution for this issue is to recreate the users profile which obviously recreates the OST file on the local PC (this fixes the problem) unfortunately this isn't a good work around when there's 20000 users... 

does anyone know of a fix for this?

Hello,

If you are having problem with Microsoft Outlook 2010 through exchange server and using cached mode you should try these steps overcome with this issue.  

1.  Close MS Outlook.
2.   Press Window Key+ R  to open run dialog box >> Type regedit  & click OK
3. Check if you have this registry key: HKCU\Software\Microsoft\Office\12.0\Outlook\Cached Mode

If not, then create that key.

Add a new DWORD value inside, name it Send One.

Set the value to 0.

Thanks & Regards

Clark Kent

• Edited by Clark445Kent 2 hours 10 minutes ago

Hello,

If you are having problem with Microsoft Outlook 2010 through exchange server and using cached mode you should try these steps overcome with this issue.  

1.  Close MS Outlook.
2.   Press Window Key+ R  to open run dialog box >> Type regedit  & click OK
3. Check if you have this registry key: HKCU\Software\Microsoft\Office\12.0\Outlook\Cached Mode

If not, then create that key.

Add a new DWORD value inside, name it Send One.

Set the value to 0.

Thanks & Regards

Clark Kent

• Edited by Clark445Kent Saturday, July 25, 2015 5:22 AM

Hello,

If you are having problem with Microsoft Outlook 2010 through exchange server and using cached mode you should try these steps overcome with this issue.  

1.  Close MS Outlook.
2.   Press Window Key+ R  to open run dialog box >> Type regedit  & click OK
3. Check if you have this registry key: HKCU\Software\Microsoft\Office\12.0\Outlook\Cached Mode

If not, then create that key.

Add a new DWORD value inside, name it Send One.

Set the value to 0.

Thanks & Regards

Clark Kent

• Edited by Clark445Kent Saturday, July 25, 2015 5:22 AM

Hi,

There are several reasons that may cause sent messages stuck in Outbox:

1.An add-in that marks items read if you view the Outbox

2.Wrong password or not authenticating properly with the mail server

3.Outlook or the mail server is offline

4.No default email account is set

5.Corrupt Send and Receive Settings

6.Using a program that accesses pst or ost data, such as Desktop Search, Lync, etc

7.An antispam or antivirus scanner is scanning outgoing mail

Please follow the link below to verify this issue can only be fixed by recreating the users profiles:

http://www.slipstick.com/problems/common-problems-that-cause-email-to-stay-in-the-outbox/

Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.

If only a new mail profile works, we can use a .prf file to modify Outlook profiles for all users. See:

https://technet.microsoft.com/en-us/library/cc179062%28v=office.14%29.aspx?f=255&MSPPError=-2147217396

Best Regards,

Steve Fan
TechNet Community Support