Email stuck in outbox

If your mail stuck in outbox there are several reason may be your internet connection is not work properly. You need to fix it first.
Another reason is that your sever may be offline so check settings for it.
Have you recently change your password for mail if yes sign out from outlook and again add your mail to outlook with new password.
All steps you can read here
https://support.microsoft.com/en-us/kb/2663435

Checks if your connection to the internet is active, can also be a problem with the Outlook mail server test it with other email accounts that same server, if malfunctioning will have to redo this mailbox

We have one mailbox server with 2 databases. Other mailboxes on this server are working fine. Outlook for users is not set to cached mode. And this mailbox is 7 GB. Could the size of this mailbox cause the problems?

The question of the size of the box has no problem've seen larger than 7 GB nehum without error, may be occurring due it is not set to the way cache.O Transfer Cached Mode takes advantage of Outlook's ability to automatically determine the speed the connection with the Exchange and optimizes data transmissions for that speed.You know how to enable the cache mode?

Have you tried to run scanpst command?

https://www.experts-exchange.com/questions/22622738/Outlook-2007-failed-to-open-cannot-find-scanpst-exe.html

Turn on Cached Exchange Mode

On the Tools menu, click Account Settings.

On the Mail tab, click the Exchange Server account, and Change.

In Microsoft Exchange Server, select the check box Use Cached Exchange Mode.

If you can not enable Cached Exchange Mode may be experiencing these problems:

You do not have an Exchange email account on your Microsoft Outlook profile This feature requires that the Outlook profile contains an Exchange account.

The Exchange administrator may have disabled this functionality Exchange administrators have the ability to disable this feature in Outlook. For more information, contact your Exchange administrator. More information administrators are available on the Microsoft Office Resource Kit.

You have installed Microsoft Terminal Services Cached Exchange Mode uses an offline folder file (.ost). This file can not be used if you have installed Microsoft Terminal Services for Microsoft Windows 2000 Server or Windows Server 2003. However, the installation of Client Terminal Services or Remote Desktop Connection client does not prevent access to this feature.

Exit and restart Office Outlook 2007.

When you are using Cached Exchange Mode, the message may remain in the Outbox for a minute, until there is the next synchronization with the server running Exchange. If you want to send the message immediately, perform the following action:

On the Tools menu, point to Send / Receive, and then click Send All.

NOTE In addition to your Exchange folders, Cached Exchange Mode works with shared folders. Examples of shared folders are someone else's Exchange folders when you are using the Delegate Access or SharePoint folders. To enable or disable support for shared folders when using the Transfer Cached Mode, after step 2 in the above procedure, click More Settings, then the Advanced tab, check or uncheck the checkbox Download shared folders ( excludes mail folders).

I'm assuming that this is hosted on a local Exchange server.  Have you tried using OWA to access the account and send the email?  This would help to identify if it is a problem with Outlook or with the server/account.

Have you cleared out the Outbox and tried sending to a different address?  I'd start with one on the same server.

Assuming this is an Exchange setup, the size of the mailbox could have everything to do with it.

Exchange server has some sanity checks built into it in terms of the number of object, folders, etc that it can deal with at one time.   When you hit these limits (and you can when Outlook is accessing multiple mailboxes), you will get very erratic operation.  I often see this when someone uses their mailbox as a filing system (one user had 5,000 folders), and then the mailbox is shared when they leave.

So as has been said, check OWA and see if you can send mail from there.  Also, check the server event looks to see if your exceeding Exchange limits.

Jim.

Thanks Jim. This being a Shared mailbox has the AD account disabled. So how would I access the mailbox in OWA.

The mailbox has following number of items:
Total Mailbox Size (Mb)            : 7238
Mailbox Size (Mb)                  : 7204
Mailbox Recoverable Item Size (Mb) : 34
Mailbox Items                      : 16496
Inbox Folder Size (Mb)             : 6361
Sent Items Folder Size (Mb)        : 822

I ran get-serverhealth and :-
On the CAS server I get these as unhealthy:
Server          State           Name                 TargetResource       HealthSetName   AlertValue ServerComp
                                                                                                                                                                         onent    
------          -----           ----                 --------------       -------------   ---------- ----------
srv-1        Online          OnPremisesSmtpCli...                      FrontendTran... Unhealthy  Fronten...
srv-1         NotApplicable   OutlookRpcCtpMonitor                      Outlook         Unhealthy  None      
srv-1         NotApplicable   ComplianceOutlook...                      Compliance      Unhealthy  None

On the Mailbox server I get these as unhealthy:    

srv-2         NotApplicable   Mapi.Submit.Monitor  MailboxTransport     MailboxTrans... Unhealthy  None  
srv-2         NotApplicable   RpsDeepTestPSProx...                      RPS.Protocol    Unhealthy  None  

Could any of these result is this issue as well?