Posts Tagged ‘NDR’

Recently a user tried to send an email from 365 to 365 user  ( external not internet ) and got the following bounce back  

Generating server: SYYP282MB0848.AUSP282.PROD.OUTLOOK.COM

Remote Server returned ‘450 4.5.0 Unable to proxy recipient (6a40c324-f14c-4772-8173-153f28bd5a97,c668bfef-4e9a-4374-9709-f7cc3ab2e31e)’

A look at the headers showed it never left 365! The email was resent and did not bounce back , there must of been an internal 365 issue

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Recently a few users had the following bounce backs from Office 365 to an On-Prem Mailserver with the error

‘550 5.4.316 Message expired, connection refused(Socket error code 10061)

Checking the logs in 365 of this, this was due to the Fortigate adding some 365 SMTP servers to the IPS Quantarine List

Removing these servers from the Quarantine and also removing IPS checking in the Policy of 365 servers to on-prem via SMTP resolved this

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