Email

Exim – Address Re-Write (Flattening Sub-Domain)

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We had an interesting issue where our EXIM configuration was flattening a subdomain’s “From” address, i.e. it was re-writing an email’s From (HFrom) and MailFrom (return-path/envelope-from) headers to @domain.com addresses, i.e. to catch emails such as root@mymachine.domain.comand re-write the From (HFrom) and MailFrom (return-path/envelope-from) to root@domain.com instead. Problem was, this configuration wasn’t documented, so needed a bit of investigation; but is also a good learning point … Read more

Microsoft 365 Exchange Online Mailbox Created Before Being Created in On-Premise Exchange

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So you created an Exchange Online User and Mailbox but you’re still in a hybrid configuration where you are managing Exchange mailboxes on-premise. What this means is that the on-premise Exchange Organisation has no idea that the user exists, it doesn’t appear in Exchange Admin Center and it also probably won’t appear in the GAL. … Read more

Microsoft Exchange Poison Queue

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The Microsoft Exchange Poison Queue is a little known queue on an on-premise Microsoft Exchange Server, maybe I’ve been lucky, but I’ve only seen emails end up here once in my time using Exchange Server which goes back to Exchange 5.5. However this week, we observed some messages going into the Poison Queue which were … Read more

Exploring DKIM Validation

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To explore how DKIM validation works first got an email in .eml format as an example email which was a known good email where the DKIM validation is OK. Meaning that both the header and the body are unchanged from when the email was first DKIM signed. There is a little tool called “dkimpy”, which … Read more

MTA-STS (Mail Transfer Agent Strict Transport Security) and TLS-RPT (TLS Reporting)

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Emails crossing the internet use secure connections encrypted using Transport Layer Security (TLS). However, there remain vulnerabilities in this method of protecting the confidentiality of emails, whereby a person-in-the-middle can trick incoming connections to send to another server and/or send information in the clear. MTA-STS is a standard designed to address these vulnerabilities and is … Read more