Office 2016, 365 exchange emails stuck in outbox until system restart

Ok this is a weird one and its been ongoing for many months, wondering if anyone has any ideas. We have 10-15 office 365 exchange email accounts and they all work perfectly fine apart from one account on one device. This particular account has intermittent issues with sending and emails just sit in the outbox. Some emails send up until a point and then it randomly stops. The machine has to be restarted and then they send immediatly upon opening outlook.

The same email account is also being used on other laptops a mac and an iphone which are NOT experiancing this issue. The issue is limited to this particular laptop and account only.

We have tried all the usual things...

  • deleting and recreating mail profiles,
  • scanning ost files for corruptions,
  • deleting ost files,
  • reformatting the laptop with windows 10,
  • disabling outgoing AV mail scanning,
  • checked quarantine and outgoing spam settings in the o365 exchange control panel.

...etc etc, but they are only temporary fixes and the problem returns anywhere within a few minuites to several days.

Initially the problem was occuring on Win 8.1 Office 2016. We replaced the hard drive in that machine and installed a fresh copy of Win 10, and a new licensed copy of office 2016 pro plus thinking that Win 8 may be the culprit. Exact same problem. So at this point I dont think it is the machine its-self as it has been completely reformatted with a new OS?...

Now upon speaking with MS they seem to refuse to support us because we have a business account, and that we need to contact our technical support or partner? We have no technical support or partner and set up the outlook 365 account ourselves so it seems we are now stuck and may have to migrate to another service.

Is it possible that something within the mailbox is causing this to happen? Should we empty the mailbox and archive all the mail?

Or could this be something to do with autodiscover failing to work on that particular account and device?

Outlook reports no errors.

Any suggestions would be hugely appricated. I have found other threads online with simlar issues but none of them seem to offer a solution other than everything we have already tried.

Thanks