Mail that you send via SMTPeter is preprocessed and then forwarded to the actual recipient. This normally happens in the blink of an eye, and the mail passes through SMTPeter in a fraction of a second.
However, sometimes SMTPeter deliberately delays your mail a little. If you send too many emails in too little time, receiving email clients might block your address. SMTPeter prevents this. The big email receivers (Google, Microsoft, Yahoo, et cetera) use special algorithms to detect strange behavior from senders. If such an algorithm notices that someone starts sending large volumes of email, they block further deliveries because this unexpected behavior could just as well be a sign that a computer has been hijacked and that someone started sending spam from it.
For SMTPeter however, it is completely normal to send large volumes of email. Email receivers do not find it strange if they see large volumes of email coming from us: there is nothing unusual about it. And on top of that, SMTPeter has special throttling algorithms to ensure that mail is always delivered at a steady rate. We know of each receiver how many emails they can process. If you send mail to SMTPeter at a higher rate than the receiver can process, we use internal buffers to delay your message a little.
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You can find this documentation page on GitHub.
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