I've got a website with a contact form, which delivers to a local ISPConfig mailbox, via another local ISPConfig mailbox (which may or may not, in fact, sometimes be the same local mailbox, sending to itself) using SMTP. These emails are getting stuck - it keeps retrying delivery until it gives up. But if I manually - using an email client, like Thunderbird - send an email from the sending mailbox to the receiving mailbox, this sends just fine. So email delivery, via SMTP, between these mailboxes does work, it just isn't working when a website attempts to do the same thing. I'm trying to work out what's different to cause one to get stuck but the other to send just fine. Is this perhaps a SpamAssassin thing? I do want to allow clients to be able to send emails via their mailbox, if they have one, from their website - as mentioned, websites often have contact forms and if they've got a website and an email with us, then they should be able to direct that contact form to our provided mailbox. Indeed, this is, for many clients, pretty much the entire reason why they've got an email with us. The web server (Apache + PHP-FPM) and email server (postfix) are on the same machine here. So this is an SMTP to 127.0.0.1 - and I've tried a "telnet 127.0.0.1 587" on that machine and the email server responds, as expected. The email is getting into the system - I can see it being queued in mail.log (and I did write some PHP code, using PHPMailer, to send an email and turned on debugging output, and the SMTP session goes entirely smoothly with a "message queued" at the end) and, of course, the fact that it repeatedly tries delivery and then gives up itself tells us that the SMTP is working. It's just that the email gets stuck. It doesn't actually get delivered and eventually I get an "Undelivered mail returned to sender" when it gives up. Yet if I send an email from the sending email to the receiving email - in the usual way, with an email client - then these emails get through without issue. So the exact same thing - other than that the point of origin is not 127.0.0.1 when I send through Thunderbird - but one works, while the other gets stuck. It is, indeed, ironically backwards. External email sails through just fine, but it's the internal emails - on 127.0.0.1 - that get stuck and never arrive, which you'd think should be easier, as it's just sending to itself. I'm wondering if it's a SpamAssassin thing, or a Postfix configuration thing. Some setting I need to tweak.