Hi, On Tue, Aug 15, 2023 at 8:49 AM Bill Cole via Postfix-users < postfix-users@postfix.org> wrote:
> On 2023-08-14 at 17:23:34 UTC-0400 (Mon, 14 Aug 2023 17:23:34 -0400) > Alex via Postfix-users <mysqlstud...@gmail.com> > is rumored to have said: > > > Hi, > > I have what appears to be a complicated mail loop problem that I can't > > figure out. I suspect that their receiving system (M365) is somehow > > reinjecting the message back to our mail server after it's been > > successfully delivered to them. > > For loose values of "success"... > > > > We are acting as MX for two small companies, and occasionally, when > > companyA emails companyB, it is first received by raven.example.com, > > 209.216.111.115, > > which is the MX we have created for them, processed by amavisd, then > > routed > > to the destination through our postfix-out instance > > xavier.example.com, > > 209.216.111.114. The companyB server accepts the message, but then > > somehow > > companyA appears to connect to our server again and send the same > > message > > again. > > Yes, it is a loop. The loop occurs inside MS365. Apparently Microsoft > does not understand how to get mail from CompanyA to CompanyB > internally, so they follow the DNS. > but it should then send it to another tenant, correct? The sending M365 server ultimately gets a "too many hops" error, reportedly by our xavier server, but we don't always have a record of that. Diagnostic information for administrators: Generating server: PH0PR02MB7736.namprd02.prod.outlook.com r...@companyb.com xavier.example.com Remote server returned '554 5.4.0 Error: too many hops' Here's one reported today: Aug 15 12:32:15 xavier postfix-out/smtp[223443]: 549A0305F4A07: to=<la...@companyb.com>, relay=companyB-com.mail.protection.outlook.com[52.101.40.2]:25, delay=2.1, delays=0.01/0/0.45/1.7, dsn=2.6.0, status=sent (250 2.6.0 < mw4pr02mb74739e55fd642380cc07b22ec2...@mw4pr02mb7473.namprd02.prod.outlook.com> [InternalId=154820686141293, Hostname= CH2PR02MB6806.namprd02.prod.outlook.com] 189859 bytes in 0.317, 583.850 KB/sec Queued mail for delivery) I can trace the queue ID here back to find the other four successful deliveries of this same message, as well as find it in my always_bcc user mbox. Thanks, Alex
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