Spam Resource: Microsoft: Multiple types of invalid user bounces

Al Iverson just released a insightful blog on Spam Resource:

Sending mail to Microsoft domains can mean you get different types of bounces back for what is effectively the same problem. Check out these three different bounces, as an example:Your message to nobodyhome@kickbox857.onmicrosoft.com couldn’t be delivered.firstname.lastname wasn’t found at company.com.550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient firstname.lastname@company.com not found by SMTP address lookup550 5.5.0 Requested action not taken: mailbox unavailable (S2017062302). [HE1EUR01FT020.eop-EUR01.prod.protection.outlook.com 2023-09-19T15:05:34.324Z 08DBB71928CA8F8E]: host company-com.mail.protection.outlook.com[52.101.40.6] said: 550 5.4.1 Recipient address rejected: Access denied. [CY4PEPF0000EE3C.namprd03.prod.outlook.com 2023-09-18T12:48:32.808Z 08DBB45899852BA7] (in reply to RCPT TO command)All of these are essentially “user unknown” or “invalid user” bounces, returned from attempting to send mail to an invalid or closed address at a Microsoft-hosted email domain.The first one is the more “plain english” version that Microsoft builds into the DSN (Delivery Status Notification) that O365 will send back in some cases.The second one? Seems self explanatory. O365 again, very clearly saying “recipient not found.”The third one? This is your

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