[Resolved] Inbound email delivery issue from Microsoft hosted email accounts

We are aware of an issue with inbound email delivery from Microsoft hosted email accounts, which is only apparent when the email includes an attachment.
The email delivery from the Microsoft server starts, but then the data transmission stalls part way through transmission, and eventually times out.
The Microsoft server will periodically re-try, and eventually the email will get through, albeit late.
Smaller messages get through straight away, but with larger messages (messages with attachments), the data transmission often stalls before the transmission is completed.
This issue is only apparent with email with attachments from Microsoft servers, and all other email providers are delivering normally.
We have made comprehensive checks on our side, and can find no issues on our servers or network which would account for this issue. We have contacted upstream network providers and asked them to make checks to try to pinpoint the source of the issue.

Posted in Mail Services, Network, Ongoing

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