Occassional email bouncing

[Tim Hall]

I've been missing out on some emails from a contact.  One bounced this morning but was sent fine again a few minutes later.

Here's the bounce message:


redacted_name@clara.co.uk
host clara-co.mail.protection.outlook.com [104.47.84.110]
SMTP error from remote mail server after RCPT TO:<redacted_name@clara.co.uk>:
550 5.4.1 Recipient address rejected: Access denied. AS(201806281) [AU2ARE01FT001.eop-are01.prod.protection.outlook.com]

What's going on?

[Kim]

Exchange is b0rked.

[Afasoas]

https://www.undocumented-features.com/2020/07/23/exchange-online-protection-550-5-4-1-recipient-address-rejected-access-denied-as201806281/

The message is getting to Exchange Online, but it is being rejected.  550 is generally a “mailbox not found,” but the status code is … interesting, as we know the mailbox exists and can successfully conduct internal transactions.

In this case, the temporary solution was disabling Directory-Based Edge Blocking (DBEB) by changing the customer domains (not the onmicrosoft.com ones) in Exchange Online to InternalRelay.

Glad my job doesn't involve exchange.