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I send emails from the web interface of Gmail Workspace (business email). Most of the clients receive my letters just fine. But some of the clients only receive messages with text and images.

Messages with links or PDF attachments do not get through. They do not return or end up in the client's spam folder. They silently go nowhere.

What could be the reason for this and how can I increase the chance to be delivered for my emails?

I have DMARC, SPF, and DKIM set up for my domain. According to the reports I receive, my emails pass the tests. I see no errors or failures for the non-delivered emails.

I tried to send all emails in plain text, but it looks like it does not affect the delivery.

EDIT:

According to Google Workspace Email Log Search all my messages get to client servers. They blocked there before dispatching to the recipients.

I checked my domain and Google Safe Browsing and Spamhaus reported no issues.

All the clients that do not receive some of my emails are using outlook.com / MS Exchange.

I sent a copy of a blocked message to my own account at Office 365 and it got to Quarantine. So, the issue is 100% reproducible now.

I checked the headers of the quarantined email and see this

X-Forefront-Antispam-Report:
 CIP:209.85.166.47;CTRY:US;LANG:en;SCL:9;SRV:;IPV:NLI;SFV:SPM;H:mail-io1-f47.google.com;PTR:mail-io1-f47.google.com;CAT:AMP;SFS:(13230001)(356005)(9686003)(336012)(86362001)(55446002)(83380400001)(42186006)(7636003)(5660300002)(7596003)(6666004)(224303003)(6916009)(1096003)(26005)(966005);DIR:INB;
X-Microsoft-Antispam: BCL:0;
X-Microsoft-Antispam-Message-Info: /* skipped */

I spent some time decoding these and it looks like spam filtering marked the message as High confidence spam (SCL:9). The category of protection policy, applied to the message is Anti-malware (CAT:AMP)

So it is clear now why message do not get to clients. But I still would like to know how to make them get there.

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    Are these missing emails counted in your incoming DMARC aggregate reports? Can you identify common traits of recipients (e.g. all hosting their mail with Microsoft)? Since links and attachments as a common property smell like spam filtering, have you checked the URLs with common malware/phishing blocklists (such as Google Safe Browsing and Spamhaus)?
    – anx
    Commented May 18, 2022 at 18:17
  • @anx I have updated my question with more info.
    – Bobrovsky
    Commented May 19, 2022 at 6:53
  • Do you have access to a Microsoft SmartScreen-enabled Browser (I do not know how to lookup their bad URL list otherwise)? Maybe they really do have a concern with the links you send..
    – anx
    Commented May 19, 2022 at 15:09
  • I tried the links with Microsoft Edge - no warnings. And they do not pass emails with PDF attachments and without links, too.
    – Bobrovsky
    Commented May 19, 2022 at 15:14

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