On Fri, Feb 20, 2026 at 6:02 PM Michael Wise via mailop <[email protected]> wrote:
> > > S3150 is what you get when you don't slow down after seeing 4xx. 😊 > If that were the case I would have SO many questions. I'll get to those in a minute, but first: that doesn't track with what I'm seeing. I have exactly two (2) 4xx SMTP errors from outlook.com mail servers in the last 5 days. But let's assume you're right... here are my top two questions: 1. How would I be expected to know that, based on available information (the SMTP error message and the documentation it points to) 2. What mail server software implements a variable outgoing rate limit based on the recent rate of 4xx messages from a receiving mail server? Surely if this is how things are intended to work, someone has thought through how senders are expected to deal with the limit.
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