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.


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Clues are slim here but try changing to a public DNS, the DNS provided by the service mentioned as well as entering the IP address of the mail server rather than its name. Keep in mind that as you manage over a dozen PCs for an office the words and terms I used here should be familiar. i.e. you are an IT pro.

commented: flattering but I am not a pro... -1

The client machine is already using public DNS and So you think it may be worth adding the account manually as opposed to using autodiscover? I dont know if O365 exchange will work with an IP as the host as this is a "cloud" based service but I will look into it... I know you can use a hostname as I have manually configured older office clients in the past.

Not sure about the IT pro comment, but yes I manage O365 installations in multiple offices for a handful of small businesses and never encountered a problem like this before that cannot be solved by creating a fresh mail profile or a repair install of office.

The IT comment is just that. You manage more than the usual so you are the pro here.

Beyond DNS and such. Remember your service may be unreliable. I can't test for that and then we have other issues where folk install Security Suites, set the security to maximum or we fine the suite interferes with normal operations.

in parting be sure the date and time is correct on the PC, routers and such.

Again, im not a pro im a web developer by trade this is more of a sideline. Never claimed to be a pro, never will. You learn something new every day in this trade.

We have wondered if it is server side... However would we not be getting the same issues on other clients? Why this particular machine only? Why is the same account not affected on other clients?

As mentioned in original post we have disabled outgoing mail scans on the client. If attempt a message trace for the stuck mails on the server I cant see them. They never make it to the server so doesnt appear to be server side filtering either.

Clocks are fine, all other services work including SSL etc on the client machine.

I shall attempt a manual config of the hosts on the client tomorrow and see how that goes.

As to other clients, the answer is no and eventually you find yourself fixing other clients.

I'm sharing what I've found to be issues and did not duplicate what you can find on the web about "emails stuck in the outbox" for this email app. That list is far too long for me to type in here so I didn't. As you are a web developer you know how to research so I'll skip sharing a search URL about that.

-> Another answer to why just this PC is that few PCs are duplicates of one another. Users are free to tweak and more. The email service could have hit some limits on sending that after some time, a reboot the limit is removed and the emails go out. You can't imagine the conversations you hear about folk that want to send hundreds of emails a day on Outlook.com or such.

I take it you completed why I noted so far so it's off to research what others have found as well. The one thing you discover with these issues is it's never the same thing over and over.

Trying now having manually entered the hosts on outlook. Will report back if it works as it will likely help others in the future but at this point im not keeping my hopes up.

Well I appricate your suggestions, im just pulling my hair out as you can imagine!

I must admit if this fails I might have to retire this machine, spent way too much time on it the last few months. It's a shame because its quite a hi spec i7 Asus laptop that cost nearly 2k back in 2014. Since replacing the hard disk with an SSD its easily got another 5 years left in it, but unless we can get outlook to work with this particular account reliably then its useless to them. Encountered some odd issues with machines over the years but this one takes the biscuit. Just cant get my head around it!

Maybe, just maybe it's time for a clean install. But what I find at times is the user repeats what they did to cause the issue (if it was user induced.)

Our oldest laptop in the office fleet is about 2008, an old Core2Duo fit with 3GB RAM, SSD and well, who said it was all that great? Anyhow it has a clean install of W10 and when an office worker uses it (don't talk to me about COVID-19 as the office shut down in 2020 and that's another story) but here we are and the users often say it's faster than what they have at home. Most likely it's because of the clean install and SSD.

We also make sure we use a nice DNS. Many IT staffers overlook this free optimization.

Good hunting on this one!

OK it was quite an undertaking but I eventually worked this out...

Basically.... There was an auth conflict when sending due to the fact that the outlook was logged into a shared mailbox and an account which had delegation for the said shared mailbox.

Upon removing the relevant delegation settings on the shared mailbox we have been able to send from this machine consistantly for the last month without issue.

I dont think this should be an issue as we have other machines operating the same setup, however in this particular instance it simply refused to play ball...

Hopefully this may help others in the future.

commented: nice post +0
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