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550 5.1.8 Access denied, bad outbound sender AS(42003)

“Your message couldn’t be delivered because you weren’t recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it’s no longer allowed to send email. Contact your email admin for assistance.” This is an error you get when your anti-spam “outbound” policy restricts […]

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[New] External Email Notification in Exchange Online

This is a new feature released in March 2021 that adds support in Outlook (Mac, OWA, Mobile) for the display of the external status of the sender – note at the time of writing it does not add this feature to Outlook for the PC. This should be used to replace the way this has […]

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Exchange Online Warning On Receipt Of New Email Sender

Released recently to no fanfare at all, Microsoft now has a SafetyTip that appears if you receive email from a first time recipient. Most often phish emails will come from an address you have never received email from before, and sometimes this email will try to impersonate people you communicate with or are internal to […]

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Enabling Better Mail Flow Security for Exchange Online

At Microsoft Ignite 2020, Microsoft announced support for MTA-STS, or Mail Transfer Agent Strict Transport Security. This is covered in RFC 8461 and it includes making TLS for mail flow to your domains mandatory whereas it is currently down to the decision of the sender. You can publish your SMTP endpoint and offer the STARTTLS […]

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Reporting on MTA-STS Failures

This article is a follow up to the Enabling Better Mail Flow Security for Exchange Online which discusses setting up MTA-STS and in this article we cover the reporting for MTA-STS. To get daily reports from each sending infrastructure to receive reports on MTA-STS you just create a DNS record in the following format: It […]

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Enable EOP Enhanced Filtering for Mimecast Users

Blog post updated March 2020 to include more specific IP ranges for all Mimecast regions and to fix an issue where the email sender is also using the same Mimecast region as yourself and the risk of SPF failures. Enhanced Filtering is a feature of Exchange Online Protection (EOP) that allows EOP to skip back […]

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Mail Flow To The Correct Exchange Online Connector

In a multi-forest Exchange Server/Exchange Online (single tenant) configuration, you are likely to have multiple inbound connectors to receive email from the different on-premises environments. There are scenarios where it is important to ensure that the correct connector is used for the inbound message rather than any of your connectors. Here is one such example. […]

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Anonymous Emails Between On-Premises and Exchange Online

When you set up Exchange Hybrid, it should configure your Exchange organizations (both on-premises and cloud) to support the fact that an email from a person in one of the organizations should appear as internal to a recipient in the other organization. In Outlook that means you will see “Display Name” at the top of […]

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Journal Rule Testing In Exchange Online

I came across two interesting oddities in journaling in Exchange Online in the last few weeks that I noticed where not really mentioned anyway (or anywhere I could find that is). The first involces routing of journal reports and the second the selection of the journal target. The journal report, that is the message that […]

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Outbound Email Via Exchange Online Protection When Using Hybrid Exchange Online

In a long term hybrid scenario, where you have Exchange Online and Exchange Server configured and mailboxes on both, internet bound email from your on-premises servers can route in two general ways. The first is outbound via whatever you had in place before you moved to Office 365. You might have configured Exchange Online to […]

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Duplicate Exchange Online and Exchange Server Mailboxes

With a hybrid Exchange Online deployment, where you have Exchange Server on-premises and Exchange Online configured in the cloud, and utilising AADConnect to synchronize the directories, you should never find that a synced user object is configured as both a mailbox in Exchange Online and a mailbox on-premises. When Active Directory is synced to Azure […]

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Enable Report Message Add-In For Office 365

There is a new add-in available for Outlook and OWA in Office 365 that can simplify spam and phishing reporting to Microsoft for content in your mailbox. I recommend rolling this add-in out to everyone in your Office 365 tenant and for Office 365 consultants to add this as part of the default steps in […]

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DMARC Quarantine Issues

I saw the following error with a client the other day when sending emails from the client to any of the Virgin Media owned consumer ISP email addresses (virginmedia.com, ntlworld.com, blueyonder.com etc.) mx3.mnd.ukmail.iss.as9143.net gave this error:vLkg1v00o2hp5bc01Lkg9w DMARC validation failed with result 3.00:quarantine In the above, the server name (…as9143.net) might change as will the value […]

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Forcing Transport Level Secure Email With Exchange Online

In Exchange Online there are a few different options for forcing email to require an encrypted connection. These depend upon the level of licence you have, and some of them are user based (Office 365 Message Encryption for example), but there are two ways to force TLS (transport layer security) for the email between when […]

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XOORG, Edge and Exchange 2010 Hybrid

So you have found yourself in the position of moving to Exchange Online from a legacy version of Exchange Server, namely Exchange 2010. You are planning to move everyone, or mostly everyone to Exchange Online and directory synchronization plays a major part (can it play a minor part?) in your plans. So you have made […]

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Malware Filter Policy Updates in Office 365

In March I wrote a blog post that showed how to take the attachment filter list from Edge Server and add those attachment block types to EOP, as EOP had a very small list of attachments. Today on one of my client tenants I noticed this precanned list of attachment extension types is now at […]