October 9, 2020 1:29 AM, "Demi M. Obenour" <demioben...@gmail.com> wrote:
> I was looking at the EuroBSDCon 2017 presentation on OpenSMTPD, and I > was wondering how it differs from the dedicated high-volume MTA that > wound up being written for the ESP. What are the features that are > needed for high volume, but otherwise don't make sense? > Hi, I should probably write an article about that, I'll keep this mail short. When you enter the realm of high-volume MTA, the rules of SMTP change and a general purpose MTA will no longer do the job: you're no longer below a radar for big ISP and big mailer corps, you're now hogging their machines and need to comply to a ton of SMTP-unrelated rules. There are limits to enforce and they are not like the generic ones we use in OpenSMTPD, they are faaaaaaar more fine-grained. For example, some are tied to your domain/IP reputation and need to be adapted dynamically, and others apply to a whole cluster of MX meaning that your MTA needs to know that domain X and Y share the same limits because they both have an MX in the same cluster and the limits apply to the whole cluster. A lot of this is unrelated to SMTP and makes the SMTP engine much much more complex. It is possible to tweak a general purpose MTA to kinda work, it just takes a ton of work. Then another issue is that when you start going above radar, you start to get feedback from ISP and big mailers ... encapsulated in SMTP responses. You can no longer just handle SMTP responses like 421 or 550, because the human message that follows will contain a destination-specific code which will provide more info (or not) about why the error happened. If your MTA is unable to handle these, then when you get a 550 from microsoft you are going to permanently fail the message when you maybe should have waited a bit to retry. With a general purpose MTA, some of your recipients are not going to receive their mail, it's that simple. With a general purpose MTA and custom tuning you can send quite a lot but there's still a limit to it. You can push that limit with good reputation but at some point, unless you're part of the big ones you'll start having issues.
