On 6/7/2021 2:08 PM, Thomas Walter via mailop wrote: (snip) > And there's also systems that send a 5xx and immediately disconnect > without waiting for the "quit" from the initiating party to properly > terminate the SMTP session. > > In those cases MTAs following the RFCs see this as a failed connection > and try again, no matter if the return code specified a permanent or > temporary issue.
This is behavior specifically allowed in RFC5321, and your sending MTA need to accommodate it. <quote> 7.8. Resistance to Attacks In recent years, there has been an increase of attacks on SMTP servers, either in conjunction with attempts to discover addresses for sending unsolicited messages or simply to make the servers inaccessible to others (i.e., as an application-level denial of service attack). While the means of doing so are beyond the scope of this Standard, rational operational behavior requires that servers be permitted to detect such attacks and take action to defend themselves. For example, if a server determines that a large number of RCPT TO commands are being sent, most or all with invalid addresses, as part of such an attack, it would be reasonable for the server to close the connection after generating an appropriate number of 5yz (normally 550) replies. </quote> _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop
