On Mon, Jan 1, 2024 at 9:31 AM Simon Arlott via mailop <mailop@mailop.org> wrote:
> On 31/12/2023 16:02, John Levine via mailop wrote: > > A message with a dozen recipients in the same SMTP session is a very > > strong spam signal. So don't do that, do single deliveries like > > everyone else does. > > Except that Google and Microsoft don't do single deliveries. Yahoo does. > > "Do as I say, not as I do"? > I have no particular insight into whether the spam false positives here would be helped/hindered by switching to single delivery, nor do I think Google has a recommendation one way or the other... but the vast majority of mail sending by Google is single delivery. While sending to multiple recipients in the same domain from Gmail can result in a single smtp transaction, there are limits on the number of recipients, and any number of other constraints will cause it to switch to using single recipient, especially workspace routing rules. Some MTAs will accept messages for multiple domains in a single inbound smtp transaction if they all share an MX, Google does not support that and will temp fail non-matching domains. There are combinations of Workspace email rules that will result in restricting the set of recipients allowed in a single inbound smtp transaction, temp failing the rest. To better handle the above, it is useful to always use single recipient, or to split when you get a mixture of success/failure on a transaction, or to retry a mix success/failure more quickly. Both of the above could potentially be solved with https://datatracker.ietf.org/doc/html/draft-hall-prdr-00 but single recipient is certainly easier. Brandon
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