Score:3

Server

X-Matching-Connectors exceeded allowed maximum

Drewes

3/3/23, 7:53 AM

When sending some mails from Postfix to Outlook365 i receive an error:

Nov  1 08:00:00 mail postfix/smtp[16252]: B7E8079FA8F:
to=<somemail.dk>,
relay=somemail.mail.protection.outlook.com[104.47.7.138]:25,
delay=0.71, delays=0.06/0/0.1/0.55, dsn=5.6.211, status=bounced (host
somemail.mail.protection.outlook.com[104.47.7.138] said: 554 5.6.211
Invalid MIME Content: Single text value size (32784) exceeded allowed
maximum (32768) for the 'X-Matching-Connectors' header.
[FR3P281MB0970.DEUP281.PROD.OUTLOOK.COM]
[AM6P192CA0016.EURP192.PROD.OUTLOOK.COM]
[BE0DEU01FT017.eop-deu01.prod.protection.outlook.com] (in reply to end
of DATA command))

To avoid this i have tried to strip all X-Matching-Connectors from my mails, but the this does not solve the problem, a matter a fact it seems like the outgoing mails does not have this header at all (i use postfix header_checks to remove another header just to make sure it works, and i can se this is removed in the log).

I also cannot find any info on the X-Matching-Connectors anywhere. Anyone know what it is and maybe where it is added?

How can I solve this problem?

Only found this online: https://answers.microsoft.com/en-us/msoffice/forum/all/getting-ndr-from-some-servers-headers-too-large/a3ace969-9d08-4d07-967a-5f40f9a0bad7

UPDATE == 5-11 ==

I have tried to set header_checks up to log ALL headers in the outgoing mail, and the offending X-Matching-Connectors is not send from Postfix to Outlook. Maybe its a header being generated in the Microsoft mailserver?

Further info : Our Postfix server is also on a Linode server (as M Klein, below). But running as a standard mailserver.

Answer to comments:

Yes, the Postfix mailserver has worked for years without this problem, and can send to gmail and other servers without issues.

Yes, I can send to the receiver from fx gmail without issues.

No, it does not seem to be all email to office365 which have this issue, only some recipients/domain. But its all mails send to these domains.

https://www.linode.com/community/questions/22063/anybody-having-issues-sending-mail-to-exchange-online-domains-from-european-loca

UPDATE == 24-11 ==

The problem seems to be related to how the mails are encoded. At lease in a test script I made.

no multipart AND charset utf-8 : FAILS (allowed maximum (32768) for the 'X-Matching-Connectors' header)
no multipart AND charset us-ascii : WORKS

multipart AND charset utf-8 : WORKS
multipart AND charset us-ascii : WORKS

470

0 + 0

outlook

postfix

Ivan_Wang

3/4/23, 1:40 AM

Could you successfully send the same message from Postfix to other email servers(e.g. outlook.com, hotmail, gmail)? What about sending from other email servers to office 365?

Ivan_Wang

3/4/23, 1:50 AM

If all messages which are sent to office 365 show this same header, check if they reach the Exchange Online limit: **Capacity alerts** & **Message limits** & **Receiving and sending limits** (https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#capacity-alerts)

Ivan_Wang

3/19/23, 1:32 AM

Hi, it's been a while, any update?

Drewes

3/23/23, 7:32 AM

No, the problem persists. I have tried to get help from Microsoft, but they insists on that the support request must come from their customers through the Microsoft Partner.

M Klein

3/25/23, 9:21 AM

We saw the exact same behaviour since roughly Oct 29th. I also tried header_checks to strip them, nothing changes. Setup: postfix on a Linode host, forwarding/sending mails to our Office 365 connector. Everything else works flawlessly since years. --------- Solution as of Nov 4th 2021 We could resolve the issue by adding an incoming connector on the exchange side, which obviously wasn't necessary until now. (and we still are not sure why it is technically necessary)

Score:1

Server

Vittal

3/17/23, 1:15 PM

We have also seen this from some of our Linode mail relays.

The problem we've found seems to be down to sending mail from European relays to Microsoft 365 accounts also hosted in Europe. If we route mail to them via the US we don't see the bounces.

We've raised this with Microsoft and have a ticket outstanding with them.

Other linode users are seeing this too https://www.linode.com/community/questions/22063/anybody-having-issues-sending-mail-to-exchange-online-domains-from-european-loca

The problem does not seem to be down to the MTA talking to Exchange (e.g. postfix). Using openssl s_connect to manually make a connection and send a minimal email also results in the same NDR.

We have found that sometimes mail can get through and the X-Matching-Connectors header is there (consisting of a lot of UUIDs).

0 + 0

ndemou

3/22/23, 8:56 AM

Hi @Vittal, how do force routing of mails through US? (I have the same problem and since Microsoft seems to drag its feet on this one I would love a workaround)

ndemou

3/22/23, 8:58 AM

BTW for me when I email a specific o365 recipient I don't always get the error. If I send a really small plain text email it passes.

Vittal

3/23/23, 9:41 AM

@ndemou We use a postfix transport_maps file (http://www.postfix.org/postconf.5.html#transport_maps) to set the SMTP relay for the affected domains to our US relay. The map file contains entries like: affected.domain.com smtp:[our.us.relay.server.ip]

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: X-Matching-Connectors exceeded allowed maximum

TH: X-Matching-Connectors เกินค่าสูงสุดที่อนุญาต

RO: X-Matching-Connectors au depășit maximul permis

RU: X-Matching-Connectors превысил допустимый максимум

VI: X-Matching-Connectors vượt quá mức tối đa cho phép

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