Here's one I hadn't seen before. A sender was hitting this deferral when trying to deliver mail to Microsoft (Hotmail/Outlook.com) domains: smtp;451 4.7.652 The mail server [x.x.x.x] has exceeded the maximum number of connections. (S3115)
What stands out here is that Microsoft is very clearly saying: you're opening too many SMTP connections. That's not a vague "too much mail" or "suspicious behavior" type of message. Instead, it's concrete, specific and useful. And I think, new. I took note of it because it's not something that I recall ever running into previously, going back for a long tim. Though I do see references to it online dating back to 2018/2019, I suspect that it's something that ESP/newsletter/other bulk senders might be running into more often nowadays.
What's going to be the root cause here? Well, sending too much mail too fast, making too many connections. Maybe you're sending a whole bunch of email volume from a new sending IP address? Maybe you skipped IP warming? Or maybe you're overwhelming their infrastructure through not limiting connections appropriately.
Microsoft, historically, limits the amount of mail you can send inbound to subscribers, with variable limits based on what sending reputation and history. New/questionable senders are able to send much less volume than established senders with a known good reputation.
What to do? Slow down. Reduce concurrency. Don't try to shovel so much email volume all at once.
- If you're new to a given sending service or IP address, build things up gradually. IP warming exists for a reason.
- And if you ARE currently engaged in IP warming or are otherwise scaling up volume, consider requesting pre-emptive accommodation from Microsoft. I explain how to do that here.
- And of course, make sure you're following Microsoft's sender guidelines. I cover that, and more, in the full MAGY (Microsoft, Apple, Gmail, Yahoo) compliance guide I put together, which you can find here.
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Comments
Thanks for this Al. I've been getting a lot of S3150 errors. Looking at the logs it seems I get one of these S3115 errors first, then a bunch of S3150 (about 25% failure rate) over the next 9 days. Then it clears.
ReplyDeleteIn the space of 20 seconds I sent 18 emails to msn/hotmail/outlook/live addresses. Doesn't really seem that excessive to me, and all were genuine emails to users who have subscribed and want them. Do you have any guidelines as to what is an appropriate rate to send them out?