When using Outlook 2013 in Exchange Cached Mode, e-mails sent via shared e-mail accounts are getting stuck in the outbox.
As soon as I disable cached mode, the mails are gone. Has someone experienced this before and fixed this?
Yes. This has been a problem since Outlook 2003. The best and only method I know to correct the problem is to turn off "Cached Mode". I believe there is an issue with Outlook synching in order to send the e-mail.
When turning it off it will require you to be online and have a decent connection to the Exchange server so be cautious of that. Another method may be to create a PST file instead which saves your e-mails locally anyway, doing away with the need for Cached Mode.