Top Deliverability’s Blog
Spam Resource: Problems sending to Microsoft over IPv6? You’re not alone
- October 26, 2022
- Posted by: Top Deliverability
- Category: Industry News Spam Resource
Al Iverson just shared a new insightful blog on Spam Resource:
Multiple folks over on the Mailop list are reporting that Microsoft OLC (Outlook.com/Hotmail/etc)’s IPv6 inbound mail servers are deferring inbound mail delivery attempts with “451 4.7.500 Server busy. Please try again later” errors. The fix seems to be to stop trying to send it over IPv6 and send the mail to any IPv4 MX record instead, and then your queues will drain successfully.What domains are affected? Likely all domains I’ve listed here as handled by Microsoft, and only if you send over IPv6, and only if the recipient domain is a Microsoft-hosted domain that has an MX record with a hostname that maps to an IPv6 address.If you’re sending mail using an ESP or CRM platform, you’re probably not affected by this. Big mail sending platforms, especially US-based ones, almost exclusively use IPv4 IP addresses, not IPv6.Why is this affecting mail over IPv6 only? Nothing has been confirmed, but I
Continued here: Problems sending to Microsoft over IPv6? You’re not alone
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