SMTP relay emails goes to spam

[gataz]

Hi all. Could anyone help me to find a solution in my case.
We use an SMTP relay to send emails from an internal application to our clients (invoices and other documents). Part of the documents goes to spam. Here is replay, which we get from clients:
DOMAINX.COM suspects your message is spam and rejected it.

We have configured spf, dkim and dmarc.

Reported error:

550 5.7.350 Remote server returned message detected as spam -> 550 X-Spam-Status: Yes, Invalid sender: DOMAINY.COM

DSN generated by:

DBAPR06MB7109.eurprd06.prod.outlook.com

What confuses me, is the DKIM part in mail header:

dkim=none (message not signed) header.d=none;dmarc=pass action=non

There are no issues if we are sending emails directly from outlook (dkim pass, spf pass, dmarc pass).

None of our domains or IP are in the spam / blacklist.

[#2]

Hi @gataz

More details required to investigate why the mails marked as spam.
You should check the full header at :
https://mha.azurewebsites.net/

If you have configured SPF and it presented as configured well in the Microsoft Message Analyzer so the problem could be the Subject / Text in the mail sent by the SMTP relay.

[#3]

Hi @Adir_Moshe 

I have tried to use this site to check header, but I can't find any useful information there. Header says that spf=pass, Spam Confidence Level 1 (which could indicate, that email don't looks like a spam message).

Thanks.

[#4]

Could you please go ahead and try change the Subject and Text that sent using the SMTP ?
For example "Scan from printer" subject can be marked as spam.

[#5]

Where is the DKIM enabled in O365 or in a third party gateway?
Do you have a partner connector created?
Can you share the header of the email in Private Message

[#6]

I can't, because this email should be sent to our customer. So the only thing I can send is invoice.

[#7]

DKIM is enabled in O365.
Connector is created.
Header has been sent in PM.
Thanks.