I'm debugging a mail relay by running it and Thunderbird locally on a Linux box. This works as expected: Thunderbird carries out an SMTP transaction to localhost:25, the relay gets the message, and passes it on. This all happens 'immediately'.

Something strange happens when I move this test setup to the real world. The relay is running on a VPS out on the net. My local Thunderbird on Windows talks to an SMTP mail server at my ISP. My ISP then looks up the mail relay, and talks SMTP to the relay.

Here's where it gets interesting. The ISP sends a DATA command to the relay, and the lines of the message. It then sends .\r\n, as expected. The relay then responds with 250 OK\r\n. The ISP then waits one minute before sending QUIT\r\n.

Any idea what's going on here, and whether I can 'fix' this? My initial thought was that my ISP (Plusnet) is just trying to rate-limit me, but it doesn't insert a delay when I send a test message through to my own mail server. So, I click 'Send' in Thunderbird, the message goes out through Plusnet, and arrives back in Thunderbird in a couple of seconds.

CodePudding user response：

Many MTA/SMTP servers cache for reuse existing SMTP connections.
Try sending a few messages a few seconds apart.

If you want more details provide name of SMTP server used by your ISP (postfix/sendmail/exim/…).