Can receive emails but not send - tricky description so please read text

[matty1961]

Up until Friday, 13th March, we were using FTTC and copper to the house, and never had ANY issues with emails at all. Full Fibre was then installed on the Friday. All appeared well until opening Outlook Classic 365, when, after a few days of using Outlook, a pop-up window informed me I wasn't connected to the internet, when I plainly was.

Having formatted my machine, the issue disappeared, but another one has reared its ugly head.

I have 4 email accounts, with one being the default account. I can send and receive emails from the other 3 accounts. The default account receives emails but won't send them. It gets bounced back thus...

Title - System Administrator

Undeliverable Testing (Testing was the title of the email)

Your message did not reach some or all of the intended recipients.

      Subject:     testing

      Sent:  21/03/2026 21:48

The following recipient(s) cannot be reached:

      [email address] on 21/03/2026 21:48

            This message could not be sent. You do not have the permission to send the message on behalf of the specified user.

It then gives me a whole page of error codes...

Diagnostic information for administrators:

__________________________________________________

Error is [0x80070005-0x000004dc-0x00000524].

Exchange response headers:

      request-id: de9eb01f-7692-106c-6ca7-1592ff4719f1

      X-ServerApplication: Exchange/15.20.9632.007

      X-FEServer: LO4P123CA0467

      X-CalculatedBETarget: AM0PR07MB5588.eurprd07.prod.outlook.com

      X-RequestId: {08EB3534-287C-4AED-AE56-24F0C175338F}:51

      X-ClientInfo: {9E2A6CE9-B57D-4C6D-8C3D-AF0B39424809}:239920042

      X-ElapsedTime: 1022

      X-BackEndHttpStatus: 200

      X-ResponseCode: 0

      X-RequestType: Execute

I have downloaded Thunderbird, and all my email accounts work. Using Outlook on the Web works, and New Outlook bundled with Windows 11 works (though I've been told that just accesses the web-based version.

Formatted my computer for the 4th time today, and I was able to send emails. I kept sending test emails. Snet on at 21:06, which was successful, but at 21:20 the email bounced back, unable to be sent.

Been onto Microsoft Support twice, the second time they want to escalate it to Level 2.

Btw, it is impossible to modify any server settings or port numbers in Outlook Classic 365 after you've added an email account.

Does anyone have any similar problems or can shed any light on this?

Many thanks.

[licquorice]

It is clearly an Outlook problem rather than BT. Outlook does seem to be problematic.

Are the other 3 accounts BTinternet accounts or other mail providers? If the latter, Outlook may be trying to send the BT mail via one of the other account's SMTP server which won't authenticate.

[matty1961]

All 4 of the email accounts are outlook.com accounts.

[licquorice]

So why on earth are you posting on a BT forum?

[matty1961]

Because BT Full Fibre is my broadband provider. It appeared to be working fine with ADSL, and I've only started having these issues since moving to Full Fibre. So the issue is either with BT or Outlook, or an incompatibility with both.

Why could I send an email perfectly at 9:06 pm, then at 9:20 pm, I sent the same test email to the same email address and got it bounced? I hadn't knowingly made ANY changes to my computer. 

It doesn't make any sense. The ONLY thing that's changed is moving from ADSL to Full Fibre.

[licquorice]

But that's like blaming the electricity company for your television not working. 

BT are just supplying a transparent pipe, there is absolutely no way they can affect just one account using a particular client.

[matty1961]

On the face of it, I agree with you.

I have another ADSL connection with the same email addresses. I shall try that soon and get back to you.

I'm thinking it's something to do with the full fibre servers being configured differently from the ADSL servers.

[licquorice]

What servers? You are connecting to an Outlook server not a BT server.

[matty1961]

Been on the BT website and raised a fault. Used the troubleshooter. 

I removed the battery back-up, I've power cycled numerous times and changed he router for a new TP AX73 and every time I get this error...

Test results

You might have some connection stability issues. This can result in your connection dropping out.


Technical details
Your hub could be in sync with the exchange or cabinet, but not connecting to the gateway. If there's no PPP session it's likely to be a problem with our network.

SO it IS an issue with the fibre connection

[licquorice]

That is an entirely different thing.

How can BT differentiate between multiple Outlook email accounts and what client you use? Totally impossible.