我已经设置了一个 Postfix + Dovecot & MySQL 服务器，用于为两个域提供电子邮件服务，每个域都有一个静态 IP。我为每个域都提供了有效的 SSL 证书，并且它们使用 .pem 格式正确安装。我所在的公司目前使用 Spambrella 进行电子邮件过滤，我想在新服务器上继续使用此服务，并且我已经使用 Spambrella 的管理门户正确配置了两个域和所有用户。
我可以使用 telnet/openssl 在端口 25、465 和 587 上连接到我的服务器，并成功向我的服务器上设置的帐户（针对两个域）发送电子邮件。IMAP（以及所需的端口）也已配置并按预期工作。
但是，每当我尝试通过 Spambrella 向我的服务器发送电子邮件时，电子邮件都会被退回。电子邮件似乎在到达我的服务器之前就被退回了，因为我没有收到退回通知。最初，我尝试按照 Spambrella 的规范为域设置 MX 记录：https://www.spambrella.com/faq/data-center-information/ 我目前已恢复 DNS MX 记录，以尽量减少电子邮件停机时间，现在我正在通过 telnet 进入 Spambrella 的服务器来测试配置：<mx1-eu1.ppe-hosted.com>，这是我们所在位置的正确服务器。我甚至多次就此问题联系过 Spambrella，他们已将其上报给他们的“高级技术团队”，但我尚未收到他们的回复。
电子邮件总是被退回，并显示以下回复：
This is the PPE Hosted Dispatch mail system.
PPE Hosted tried to deliver your email message,
but was unable to do so for reasons outside our control.
We recommend contacting the person or service responsible for
looking after the recipient domain's email server directly.
The last error message we have available from the recipient's
server is included right below, along with an extract of your
email:
<user@domain>: host a1.mail.domain.localhost[correct ip] said:
550 Relaying denied (in reply to RCPT TO command)
Reporting-MTA: dns; dispatch1-eu1.ppe-hosted.com
X-PPE-Hosted-ESMTP-Server-Queue-ID: 1578A20052
X-PPE-Hosted-ESMTP-Server-Sender: rfc822; myemail@mydomain
Arrival-Date: Fri, 18 Sep 2020 11:50:00 +0000 (UTC)
Final-Recipient: rfc822; user@domain
Original-Recipient: rfc822; user@domain
Action: failed
Status: 5.0.0
Remote-MTA: dns; a1.mail.domain.localhost
Diagnostic-Code: smtp; 550 Relaying denied
Return-Path: <myemail@mydomain>
Received: from pure.maildistiller.com (unknown [10.70.45.17])
by dispatch1-eu1.ppe-hosted.com (PPE Hosted ESMTP Server) with ESMTP id 1578A20052
for <user@domain>; Fri, 18 Sep 2020 11:50:00 +0000 (UTC)
Received: from eu1-mdac16-8.fra.mdlocal (unknown [10.70.45.28])
by pure.maildistiller.com (PPE Hosted ESMTP Server) with ESMTP id 13BD18009B
for <user@domain>; Fri, 18 Sep 2020 11:50:00 +0000 (UTC)
X-Virus-Scanned: Proofpoint Essentials engine
Received: from mx1-eu1.ppe-hosted.com (unknown [10.70.45.116])
by pure.maildistiller.com (PPE Hosted ESMTP Server) with ESMTPS id 583DC4005A
for <user@domain>; Fri, 18 Sep 2020 11:49:59 +0000 (UTC)
Received: from mydomain (my.domain [my ip])
by mx1-eu1.ppe-hosted.com (PPE Hosted ESMTP Server) with ESMTP id 75F9CB80064
for <user@domain>; Fri, 18 Sep 2020 11:49:39 +0000 (UTC)
From: <myemail@mydomain>
To: <user@domain>
Subject: This is a test message that hopefully passes DMARC
Date: Fri, 18 Sep 2020 12:12:00 -0000
X-MDID: 1600429799-WQVTD-u02kSV
X-PPE-DISP: 1600429799;WQVTD-u02kSV
Spambrella 要求所有目标服务器都使用端口 25。我一直在使用端口 25 进行测试。
如果需要的话，我可以发布我的 Postfix 配置文件，当我尝试将它们附加到这个帖子时，Stack Exchange 将它们标记为垃圾邮件。
如果有人能帮助我解决这个问题我将不胜感激。