421 Too many SMTP sessions for this host

This is not the message I wanted to get after sending an important e-mail to a supplier in Taiwan. To make things worse, this isn’t a message I received when connecting to my (e-mail) provider’s mail server. Instead, the mail server itself received this message when trying to relay my e-mail to Hinet, where my supplier has his account. Maybe somebody at Hinet had a brilliant idea of how to reduce SPAM (“let’s not accept more than 8 e-mails per month from other mail servers”) or their server was having temporary problems and nobody could be bothered to check if the error message sent together with error code 421 made sense. In any case, my e-mail wasn’t getting through.

Other than sending a fax, the solution I found was to connect directly to the mail server at Hinet (ms12a.hinet.net) from my e-mail software, thereby bypassing my provider’s mail server. On this illustration from Wikipedia, this would mean drawing an arrow from “Alice’s MUA” directly to “mx.b.org”. My first attempt failed (“relaying denied”) because I had forgotten to remove my own address from the BCC field. Since Hinet’s server (at least this one) isn’t an open relay, it only accepts e-mails for its own users (*@hinet.net). After I fixed this, sending the e-mail was no problem (and it actually arrived, too).

Luckily, it was easy to add an alternative mail server in my e-mail software (Becky!). Still, this wasn’t the first time I had experienced problems communicating with business partners in Taiwan who were using Hinet and I’m sure I’m not the only one. I’m really wondering how many potential deals were never concluded because of Hinet’s poor performance.

One thought on “421 Too many SMTP sessions for this host”

  1. 9 years later, these guys are doing the same.
    Just had to troubleshoot a mail delivery issue and found your article.

    Cheers!

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