O365 filtering out SPAM before it gets to Sophos

Hi Stuart,

Thank you for reaching out to Sophos Community and for sharing the case#.

Will further check this and apologies for the experience. 

An update - it's now been two weeks.

Microsoft say the email goes to Sophos first and Sophos say the email goes to Microsoft first.

Sophos says log a call with Microsoft

Microsoft says log a call with Sophos

So now what?

Hello Stuart,

Reviewing your case, I don't see any note saying that the "deployment doesn’t work properly."

It looks like you want Sophos Email to catch the email first, do the anti-spam checks and send it to O365, the way Gateway mode works, rather than Email Flow, where the email arrives first to O365 and sends the mail to Sophos Email (Sophos Email does the spam checks) Sophos email then send it back to O365.

Today you have a call with Support to go through this change so the email flow will now be Email > Sophos Email > o365.

Regards,

No, I do not want to use Gateway mode. The Sophos "recommended" deployment for O365 is Mailflow mode, and that's what I want to use. Sophos automatically connected to the O365 tenant and configured all the mailflow rules, and then did a test to confirm everything is working correctly, but it's not, emails are being filtered by Microsoft and all my users are getting quarantine emails from Microsoft AND Sophos. But after more than a week of investigation, Sophos' advice is they can't get it working so let's change to Gateway mode

I would much rather Sophos find and fix the problem so that their "recommended" solution worked properly. I don't have much confidence in the support team if after more than a week they throw their hands in the air and say too hard try switching to gateway mode.

Are they showing as High Confidence Phish in the M365 quarantine?

Secure by Default is something that Microsoft implemented that cannot be bypassed. The Pre-Filter rule turn the Spam Confidence Level score (SCL) to -1 which tells M365 not to inspect for spam. We are in discussions with Microsoft Product Team about this topic among others. Do you see the SCL -1 in the headers of the messages? The Pre-Filter should be the first rule in the list of rules.

Thanks Tom Foucha. It's disappointing that the support team didn't have this information a week and a half ago  - it would have saved a lot of time.

It would have been good to know before we went and implemented the Sophos recommended deployment method that doesn't work. We just followed the Sophos recommendations. Probably a good idea for Sophos to not allow new customers to use Mailflow mode until this is fixed.

There is no SCL score in the headers at all. The pre-filter is the first rule, as per screenshots above (priority 0). Microsoft lists the quarantine reason as "Spam"

Stuart, it does work, I personally have been using it for the last 16 months, Microsoft will if you make enough noise and provide business justification turn off the Secure by Default but I've only had a couple customers get that done. If there is no SCL in the headers I'd investigate a bit more. How long have you had this implementation? 

Can you check a couple of things for me: Since each M365 plan has different levels of protection this may/may not apply

In security.microsoft.com > policies & rules > threat policies > preset security policies are the Standard and Strict Protection turned on. Have you configured any Exclusions?

We implemented Sophos Mail about a month ago. The O365 tenant itself is about 10 years old.

Have checked that area and nothing has been configured

Thanks, one last section to inspect, keeping in mind that Sophos is in talks with Microsoft we are dependent on their service no matter whether it is gateway mode or MFR. Now what is said that if the MX does not point to Microsoft that you can disable some things in Secure by Default: https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/secure-by-default?view=o365-worldwide

"Third-party filters: Secure by default only applies when the MX record for your domain is set to Exchange Online Protection (contoso.mail.protection.outlook.com). If it's set to another service or device, it's possible to override"

It does become a trade off at this time between having the MX pointed to Sophos or M365. Even in my configurations I still end up with HCP (high confidence phish and a splattering of spam) ending up in the Microsoft Quarantine. 

Policies > Rules > Anti-Spam policies - inspect each policy and edit to turn off anything you can

Before moving to Sophos, we used N-Able Mail Assure, and none of the users received a Microsoft Quarantine email ever, so there's no configuration issues with the tenant config. It is frustrating that Sophos knows of this issue (hence in talks with Microsoft) yet still recommends using Mailflow mode - otherwise I would have implemented with Gateway mode to begin with.

What are the steps now to switch from Mailflow mode to Gateway mode?

And was your MX record pointing to someplace other than Microsoft? Microsoft has made changes recently since 2023 to combat bad actors creating connectors, dumping spam and tearing down things so they have become stricter on their partners. We are building a certificate based connector system because they are changing the way connectors work. 

To switch to Gateway mode. You must disconnect the MFR configuration, configure in Domain Settings, change your MX, inspect/update SPF, DKIM, DMARC records. I would suggest your work with your partner or professional service org as your environment is likely different than others.

Email Security - Domains settings/status

1. Overview
2.  Email Security Dashboard
3. Global Settings
4. Domains settings/status

https://docs.sophos.com/central/customer/help/en-us/ManageYourProducts/EmailSecurity/SophosGateway/ExternalServices/ConfigureM365/index.html#configure-a-secure-connector-between-microsoft-365-and-sophos-gateway

And was your MX record pointing to someplace other than Microsoft? Microsoft has made changes recently since 2023 to combat bad actors creating connectors, dumping spam and tearing down things so they have become stricter on their partners. We are building a certificate based connector system because they are changing the way connectors work. 

To switch to Gateway mode. You must disconnect the MFR configuration, configure in Domain Settings, change your MX, inspect/update SPF, DKIM, DMARC records. I would suggest your work with your partner or professional service org as your environment is likely different than others.

Email Security - Domains settings/status

1. Overview
2.  Email Security Dashboard
3. Global Settings
4. Domains settings/status

https://docs.sophos.com/central/customer/help/en-us/ManageYourProducts/EmailSecurity/SophosGateway/ExternalServices/ConfigureM365/index.html#configure-a-secure-connector-between-microsoft-365-and-sophos-gateway