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Are you using Cached exchange mode? If so try to disable it and see if it works. Or at least try to select not to cached shared folders in cached mode.
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Are you using Cached exchange mode? If so try to disable it and see if it works. Or at least try to select not to cached shared folders in cached mode.

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This one particular user is set up slightly differently to everyone else. Shes a PA to the CEO - call her A.<br />
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She logs in to the network as A and has her own mailbox. However, she full access to CEO mailbox and user B.<br />
Her outlook is set up as default CEO, with A and B also. She rarely uses her own A mailbox.<br />
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Most of the day she SENDS AS CEO. <br />
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Intermittently, emails get stuck and remain in the outbox. Sometimes if you go into email and send again it works (but leaves copy behind in outbox). Sometimes not.<br />
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Also, new emails SEND AS CEO go into CEO SENT items. Emails replied to (even though SEND AS CEO) go into user A SENT items.<br />
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Tried new profile. Still the same.
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This one particular user is set up slightly differently to everyone else. Shes a PA to the CEO - call her A.

She logs in to the network as A and has her own mailbox. However, she full access to CEO mailbox and user B.
Her outlook is set up as default CEO, with A and B also. She rarely uses her own A mailbox.

Most of the day she SENDS AS CEO. 

Intermittently, emails get stuck and remain in the outbox. Sometimes if you go into email and send again it works (but leaves copy behind in outbox). Sometimes not.

Also, new emails SEND AS CEO go into CEO SENT items. Emails replied to (even though SEND AS CEO) go into user A SENT items.

Tried new profile. Still the same.

Emails stuck in outbox at random - Exchange 2013/Outlook 2010

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.

Outlook

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.