Remote Server returned ‘550 5.7.708 Service unavailable (SMTP)

Question

Remote Server returned ‘550 5.7.708 Service unavailable (SMTP)

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Hello,

I am experiencing an issue sending emails from a Microsoft 365 / Exchange Online account. I have already tried practically every relevant configuration and troubleshooting step I have been able to find, but I continue to receive the same error.

The mailbox is also being used by a WordPress website through FluentSMTP, connected to Microsoft 365 / Exchange.

The message is accepted by Microsoft initially, but it is later rejected when Microsoft attempts to deliver it to an external recipient. The error I receive is:

550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. AS(7910)

The server generating the error is a Microsoft server:

MA3P292MB0189.ESPP292.PROD.OUTLOOK.COM

I have already performed the following checks and configuration changes:

I configured and reviewed the DKIM records from:

https://security.microsoft.com/authentication?viewid=DKIM

In the Microsoft 365 Admin Center:

Home > Active users > User > Mail > Manage email apps

I enabled:

Authenticated SMTP

This option was disabled by default. I have also tested with it disabled again, but the result was the same.

In Microsoft Entra Admin Center:

Overview > Properties > Manage security defaults

I disabled:

Security Defaults

to make sure they were not blocking SMTP authentication.

In the Exchange Admin Center:

https://admin.cloud.microsoft/exchange#/settings

I went to:

Mail flow > Security

and also tested by disabling the option:

Turn off SMTP AUTH protocol for your organization

This did not resolve the issue either, so I ultimately left the setting enabled again.

The domain also has a valid SPF record for Microsoft 365:

v=spf1 include:spf.protection.outlook.com -all

I have also submitted a delisting request, but this did not resolve the problem.

I have tested sending from different devices and from different public IP addresses, but the issue remains exactly the same.

Despite all of these checks, the problem continues unchanged.

SMTP authentication itself appears to be working correctly because the messages are accepted into Microsoft infrastructure and the headers show Microsoft *.PROD.OUTLOOK.COM servers. The failure appears to happen afterwards, when Microsoft attempts to deliver the message to an external organization.

For example, when sending to a Gmail address, I receive:

Remote server returned '550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. AS(7910)'

There are also several unusual details that may help identify the issue:

Messages sent to the same mailbox that is sending the email are delivered successfully (from WordPress to Outlook).

Messages sent to external organizations consistently fail, regardless of whether the recipient uses Outlook/Microsoft 365, Gmail, or another independent mail provider.

The failed messages generate bounce notifications that arrive back in the sender's own mailbox, but these bounce messages are automatically classified as spam/junk.

Sending emails normally from Outlook itself works correctly in same or diferent organizations.

The issue only occurs when messages are sent through the server / SMTP integration.

Because of this, I suspect there may be some kind of tenant-level restriction, outbound spam restriction, reputation block, or limitation affecting Microsoft 365 outbound mail infrastructure.

Could you please advise what else I should check?

I have already reviewed SMTP AUTH, DKIM, SPF, Security Defaults, Exchange Online settings, delisting procedures, and sending from different devices and IP addresses, but I still cannot successfully send external messages through the server.

Note: The domain was bought a few days ago, and the Outlook account with the licence also a day ago. And also, the domain use in the outlook account was verified a few hours ago too. I've read this might be a problem for fresh/untrusted new accounts. Anyway if this is the main cause... why just dont notify the user? Honestly I've spent all day just for this and Im really frustrated right now.

Thank you in advance for your help.

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1 answer

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Answer 1

Dear Can,

I'm sorry to hear you've spent the whole day on this. Hitting the same bounce after working through every setting is frustrating, and your suspicion about the new tenant is very likely correct. Unfortunately, as a forum moderator, I am not able to review your tenant's outbound reputation or apply an exception to fix this directly. My role here is limited to providing general guidance and solutions that can be applied by users.

Error 550 5.7.708 means Exchange Online is refusing to send the message out because the sending IP has a low reputation. Microsoft notes this is more likely to affect new customers, which fits a domain and tenant that are only a few days old. The IP in question belongs to Microsoft's outbound infrastructure, not your own connection. That's why changing devices and public IP addresses made no difference. This also means the settings you checked weren't the cause. No setting in Exchange Online or Defender clears this block. The delisting portal handles a different error, 5.7.511, which is why your request there had no effect.

The next step is to open a service request from the Microsoft 365 admin center. For detailed instructions on how to get support, please refer to Get support - Microsoft 365 admin. Ask for an IP address exception for error 5.7.708. To speed things up, include the following:

The full NDR text
The affected sending mailbox, and confirmation that an Exchange Online licence is assigned
That the messages are sent through SMTP AUTH from a WordPress plugin, while sending from Outlook directly works fine

For reference: NDR error codes 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750

Two things are worth knowing while this is resolved. First, tenants under 31 days old get only 10% of the normal daily limit for external recipients, so keep the website's sending volume low for now. Second, since Security Defaults weren't the cause here, I'd suggest turning them back on once mail flow is working, because a new tenant is more exposed without them.

Once support has applied the exception, feel free to update the thread if anything else comes up.

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